"git log rev^..rev" is commonly used to show all work done on and merged
from a side branch. This patch introduces a shorthand "rev^-" for this
and additionally allows "rev^-$n" to mean "reachable from rev, excluding
what is reachable from the nth parent of rev". For example, for a
two-parent merge, you can use rev^-2 to get the set of commits which were
made to the main branch while the topic branch was prepared.

Signed-off-by: Vegard Nossum <vegard.nos...@oracle.com>

---
[v2: Use ^- instead of % as suggested by Junio Hamano and use some
 common helper functions for parsing.]

[v3: Use 'struct object_id' instead of 'char[20]' and add some tests as
 suggested by Matthieu Moy; fix missing '-' in Documentation/revisions.txt
 as suggested by Ramsay Jones; misc changelog + documentation fixes as
 suggested by Philip Oakley.]

[v4: Documentation fixes and parsing rework suggested by Junio Hamano
 and add some more tests.]

[v5: count parents before showing anything, misc testing changes, and
 changelog shortening as suggested by Junio Hamano, parent counting
 changes suggested by Jeff King.]
---
 Documentation/revisions.txt  | 17 +++++++-
 builtin/rev-parse.c          | 54 +++++++++++++++++++------
 revision.c                   | 34 ++++++++++++++--
 t/t6101-rev-parse-parents.sh | 94 ++++++++++++++++++++++++++++++++++++++++++++
 4 files changed, 180 insertions(+), 19 deletions(-)

diff --git a/Documentation/revisions.txt b/Documentation/revisions.txt
index 4bed5b1..ba11b9c 100644
--- a/Documentation/revisions.txt
+++ b/Documentation/revisions.txt
@@ -283,7 +283,7 @@ empty range that is both reachable and unreachable from 
HEAD.
 
 Other <rev>{caret} Parent Shorthand Notations
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-Two other shorthands exist, particularly useful for merge commits,
+Three other shorthands exist, particularly useful for merge commits,
 for naming a set that is formed by a commit and its parent commits.
 
 The 'r1{caret}@' notation means all parents of 'r1'.
@@ -291,8 +291,15 @@ The 'r1{caret}@' notation means all parents of 'r1'.
 The 'r1{caret}!' notation includes commit 'r1' but excludes all of its parents.
 By itself, this notation denotes the single commit 'r1'.
 
+The '<rev>{caret}-{<n>}' notation includes '<rev>' but excludes the <n>th
+parent (i.e. a shorthand for '<rev>{caret}<n>..<rev>'), with '<n>' = 1 if
+not given. This is typically useful for merge commits where you
+can just pass '<commit>{caret}-' to get all the commits in the branch
+that was merged in merge commit '<commit>' (including '<commit>'
+itself).
+
 While '<rev>{caret}<n>' was about specifying a single commit parent, these
-two notations consider all its parents. For example you can say
+three notations also consider its parents. For example you can say
 'HEAD{caret}2{caret}@', however you cannot say 'HEAD{caret}@{caret}2'.
 
 Revision Range Summary
@@ -326,6 +333,10 @@ Revision Range Summary
   as giving commit '<rev>' and then all its parents prefixed with
   '{caret}' to exclude them (and their ancestors).
 
+'<rev>{caret}-{<n>}', e.g. 'HEAD{caret}-, HEAD{caret}-2'::
+       Equivalent to '<rev>{caret}<n>..<rev>', with '<n>' = 1 if not
+       given.
+
 Here are a handful of examples using the Loeliger illustration above,
 with each step in the notation's expansion and selection carefully
 spelt out:
@@ -339,6 +350,8 @@ spelt out:
    C                            I J F C
    B..C   = ^B C                C
    B...C  = B ^F C              G H D E B C
+   B^-    = B^..B
+         = ^B^1 B              E I J F B
    C^@    = C^1
          = F                   I J F
    B^@    = B^1 B^2 B^3
diff --git a/builtin/rev-parse.c b/builtin/rev-parse.c
index 76cf05e..4da1f1d 100644
--- a/builtin/rev-parse.c
+++ b/builtin/rev-parse.c
@@ -298,14 +298,30 @@ static int try_parent_shorthands(const char *arg)
        unsigned char sha1[20];
        struct commit *commit;
        struct commit_list *parents;
-       int parents_only;
-
-       if ((dotdot = strstr(arg, "^!")))
-               parents_only = 0;
-       else if ((dotdot = strstr(arg, "^@")))
-               parents_only = 1;
-
-       if (!dotdot || dotdot[2])
+       int parent_number;
+       int include_rev = 0;
+       int include_parents = 0;
+       int exclude_parent = 0;
+
+       if ((dotdot = strstr(arg, "^!"))) {
+               include_rev = 1;
+               if (dotdot[2])
+                       return 0;
+       } else if ((dotdot = strstr(arg, "^@"))) {
+               include_parents = 1;
+               if (dotdot[2])
+                       return 0;
+       } else if ((dotdot = strstr(arg, "^-"))) {
+               include_rev = 1;
+               exclude_parent = 1;
+
+               if (dotdot[2]) {
+                       char *end;
+                       exclude_parent = strtoul(dotdot + 2, &end, 10);
+                       if (*end != '\0' || !exclude_parent)
+                               return 0;
+               }
+       } else
                return 0;
 
        *dotdot = 0;
@@ -314,12 +330,24 @@ static int try_parent_shorthands(const char *arg)
                return 0;
        }
 
-       if (!parents_only)
-               show_rev(NORMAL, sha1, arg);
        commit = lookup_commit_reference(sha1);
-       for (parents = commit->parents; parents; parents = parents->next)
-               show_rev(parents_only ? NORMAL : REVERSED,
-                               parents->item->object.oid.hash, arg);
+       if (exclude_parent &&
+           exclude_parent > commit_list_count(commit->parents)) {
+               *dotdot = '^';
+               return 0;
+       }
+
+       if (include_rev)
+               show_rev(NORMAL, sha1, arg);
+       for (parents = commit->parents, parent_number = 1;
+            parents;
+            parents = parents->next, parent_number++) {
+               if (exclude_parent && parent_number != exclude_parent)
+                       continue;
+
+               show_rev(include_parents ? NORMAL : REVERSED,
+                        parents->item->object.oid.hash, arg);
+       }
 
        *dotdot = '^';
        return 1;
diff --git a/revision.c b/revision.c
index 969b3d1..b37dbec 100644
--- a/revision.c
+++ b/revision.c
@@ -1289,12 +1289,14 @@ void add_index_objects_to_pending(struct rev_info 
*revs, unsigned flags)
        }
 }
 
-static int add_parents_only(struct rev_info *revs, const char *arg_, int flags)
+static int add_parents_only(struct rev_info *revs, const char *arg_, int flags,
+                           int exclude_parent)
 {
        unsigned char sha1[20];
        struct object *it;
        struct commit *commit;
        struct commit_list *parents;
+       int parent_number;
        const char *arg = arg_;
 
        if (*arg == '^') {
@@ -1316,7 +1318,15 @@ static int add_parents_only(struct rev_info *revs, const 
char *arg_, int flags)
        if (it->type != OBJ_COMMIT)
                return 0;
        commit = (struct commit *)it;
-       for (parents = commit->parents; parents; parents = parents->next) {
+       if (exclude_parent &&
+           exclude_parent > commit_list_count(commit->parents))
+               return 0;
+       for (parents = commit->parents, parent_number = 1;
+            parents;
+            parents = parents->next, parent_number++) {
+               if (exclude_parent && parent_number != exclude_parent)
+                       continue;
+
                it = &parents->item->object;
                it->flags |= flags;
                add_rev_cmdline(revs, it, arg_, REV_CMD_PARENTS_ONLY, flags);
@@ -1519,17 +1529,33 @@ int handle_revision_arg(const char *arg_, struct 
rev_info *revs, int flags, unsi
                }
                *dotdot = '.';
        }
+
        dotdot = strstr(arg, "^@");
        if (dotdot && !dotdot[2]) {
                *dotdot = 0;
-               if (add_parents_only(revs, arg, flags))
+               if (add_parents_only(revs, arg, flags, 0))
                        return 0;
                *dotdot = '^';
        }
        dotdot = strstr(arg, "^!");
        if (dotdot && !dotdot[2]) {
                *dotdot = 0;
-               if (!add_parents_only(revs, arg, flags ^ (UNINTERESTING | 
BOTTOM)))
+               if (!add_parents_only(revs, arg, flags ^ (UNINTERESTING | 
BOTTOM), 0))
+                       *dotdot = '^';
+       }
+       dotdot = strstr(arg, "^-");
+       if (dotdot) {
+               int exclude_parent = 1;
+
+               if (dotdot[2]) {
+                       char *end;
+                       exclude_parent = strtoul(dotdot + 2, &end, 10);
+                       if (*end != '\0' || !exclude_parent)
+                               return -1;
+               }
+
+               *dotdot = 0;
+               if (!add_parents_only(revs, arg, flags ^ (UNINTERESTING | 
BOTTOM), exclude_parent))
                        *dotdot = '^';
        }
 
diff --git a/t/t6101-rev-parse-parents.sh b/t/t6101-rev-parse-parents.sh
index 1c6952d..64a9850 100755
--- a/t/t6101-rev-parse-parents.sh
+++ b/t/t6101-rev-parse-parents.sh
@@ -102,4 +102,98 @@ test_expect_success 'short SHA-1 works' '
        test_cmp_rev_output start "git rev-parse ${start%?}"
 '
 
+# rev^- tests; we can use a simpler setup for these
+
+test_expect_success 'setup for rev^- tests' '
+       test_commit one &&
+       test_commit two &&
+       test_commit three &&
+
+       # Merge in a branch for testing rev^-
+       git checkout -b branch &&
+       git checkout HEAD^^ &&
+       git merge -m merge --no-edit --no-ff branch &&
+       git checkout -b merge
+'
+
+# The merged branch has 2 commits + the merge
+test_expect_success 'rev-list --count merge^- = merge^..merge' '
+       git rev-list --count merge^..merge >expect &&
+       echo 3 >actual &&
+       test_cmp expect actual
+'
+
+# All rev^- rev-parse tests
+
+test_expect_success 'rev-parse merge^- = merge^..merge' '
+       git rev-parse merge^..merge >expect &&
+       git rev-parse merge^- >actual &&
+       test_cmp expect actual
+'
+
+test_expect_success 'rev-parse merge^-1 = merge^..merge' '
+       git rev-parse merge^1..merge >expect &&
+       git rev-parse merge^-1 >actual &&
+       test_cmp expect actual
+'
+
+test_expect_success 'rev-parse merge^-2 = merge^2..merge' '
+       git rev-parse merge^2..merge >expect &&
+       git rev-parse merge^-2 >actual &&
+       test_cmp expect actual
+'
+
+test_expect_success 'rev-parse merge^-0 (invalid parent)' '
+       test_must_fail git rev-parse merge^-0
+'
+
+test_expect_success 'rev-parse merge^-3 (invalid parent)' '
+       test_must_fail git rev-parse merge^-3
+'
+
+test_expect_success 'rev-parse merge^-^ (garbage after ^-)' '
+       test_must_fail git rev-parse merge^-^
+'
+
+test_expect_success 'rev-parse merge^-1x (garbage after ^-1)' '
+       test_must_fail git rev-parse merge^-1x
+'
+
+# All rev^- rev-list tests (should be mostly the same as rev-parse; the reason
+# for the duplication is that rev-parse and rev-list use different parsers).
+
+test_expect_success 'rev-list merge^- = merge^..merge' '
+       git rev-list merge^..merge >expect &&
+       git rev-list merge^- >actual &&
+       test_cmp expect actual
+'
+
+test_expect_success 'rev-list merge^-1 = merge^1..merge' '
+       git rev-list merge^1..merge >expect &&
+       git rev-list merge^-1 >actual &&
+       test_cmp expect actual
+'
+
+test_expect_success 'rev-list merge^-2 = merge^2..merge' '
+       git rev-list merge^2..merge >expect &&
+       git rev-list merge^-2 >actual &&
+       test_cmp expect actual
+'
+
+test_expect_success 'rev-list merge^-0 (invalid parent)' '
+       test_must_fail git rev-list merge^-0
+'
+
+test_expect_success 'rev-list merge^-3 (invalid parent)' '
+       test_must_fail git rev-list merge^-3
+'
+
+test_expect_success 'rev-list merge^-^ (garbage after ^-)' '
+       test_must_fail git rev-list merge^-^
+'
+
+test_expect_success 'rev-list merge^-1x (garbage after ^-1)' '
+       test_must_fail git rev-list merge^-1x
+'
+
 test_done
-- 
2.10.0.rc0.1.g07c9292

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