[PATCH v2 5/6] rebase --rebase-merges: a "merge" into a new root is a fast-forward

[Johannes Schindelin]

When a user provides a todo list containing something like

        reset [new root]
        merge my-branch

let's do the same as if pulling into an orphan branch: simply
fast-forward.

Signed-off-by: Johannes Schindelin <johannes.schinde...@gmx.de>
---
 sequencer.c              | 12 ++++++++++++
 t/t3430-rebase-merges.sh | 13 +++++++++++++
 2 files changed, 25 insertions(+)

diff --git a/sequencer.c b/sequencer.c
index a7832399b1f..65a8c493781 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -2870,6 +2870,18 @@ static int do_merge(struct commit *commit, const char 
*arg, int arg_len,
                goto leave_merge;
        }
 
+       if (opts->have_squash_onto &&
+           !oidcmp(&head_commit->object.oid, &opts->squash_onto)) {
+               /*
+                * When the user tells us to "merge" something into a
+                * "[new root]", let's simply fast-forward to the merge head.
+                */
+               rollback_lock_file(&lock);
+               ret = fast_forward_to(&merge_commit->object.oid,
+                                      &head_commit->object.oid, 0, opts);
+               goto leave_merge;
+       }
+
        if (commit) {
                const char *message = get_commit_buffer(commit, NULL);
                const char *body;
diff --git a/t/t3430-rebase-merges.sh b/t/t3430-rebase-merges.sh
index 35260862fcb..5543f1d5a34 100755
--- a/t/t3430-rebase-merges.sh
+++ b/t/t3430-rebase-merges.sh
@@ -275,4 +275,17 @@ test_expect_success 'root commits' '
        test_cmp_rev HEAD $before
 '
 
+test_expect_success 'a "merge" into a root commit is a fast-forward' '
+       head=$(git rev-parse HEAD) &&
+       cat >script-from-scratch <<-EOF &&
+       reset [new root]
+       merge $head
+       EOF
+       test_config sequence.editor \""$PWD"/replace-editor.sh\" &&
+       test_tick &&
+       git rebase -i -r HEAD^ &&
+       test_cmp_rev HEAD $head
+'
+
+
 test_done
-- 
2.17.0.windows.1.38.g05ca542f78d