The current behavior is inconsistent when passing SHA-1 to get_sha1.
If it's a short sha-1, refs take precedence. "git rev-parse 1234" will
resolve refs/heads/1234 if exists even if there is an unambiguous
SHA-1 starting with 1234. However if it's full SHA-1, the SHA-1 takes
precedence and refs with the same name are ignored.

The former makes more sense than the latter. This patch makes git
check for 40-hex ref names before consider it SHA-1. In future, we may
want to warn ambiguity between refs and SHA-1 (for both full and short
SHA-1).

Signed-off-by: Nguyễn Thái Ngọc Duy <pclo...@gmail.com>
---
 Some gray area where my "makes more sense" may not actually be common
 sense.

 sha1_name.c                         | 13 +++++++++++--
 t/t1512-rev-parse-disambiguation.sh | 15 +++++++++++++++
 2 files changed, 26 insertions(+), 2 deletions(-)

diff --git a/sha1_name.c b/sha1_name.c
index 3820f28..faf10b4 100644
--- a/sha1_name.c
+++ b/sha1_name.c
@@ -436,11 +436,20 @@ static int get_sha1_basic(const char *str, int len, 
unsigned char *sha1)
 {
        static const char *warn_msg = "refname '%.*s' is ambiguous.";
        char *real_ref = NULL;
-       int refs_found = 0;
+       int refs_found;
        int at, reflog_len;
 
-       if (len == 40 && !get_sha1_hex(str, sha1))
+       if (len == 40 && !get_sha1_hex(str, sha1)) {
+               unsigned char ref_sha1[20];
+               refs_found = dwim_ref(str, len, ref_sha1, &real_ref);
+               if (refs_found > 0) {
+                       if (warn_ambiguous_refs && refs_found > 1)
+                               warning(warn_msg, len, str);
+                       hashcpy(sha1, ref_sha1);
+               }
+               free(real_ref);
                return 0;
+       }
 
        /* basic@{time or number or -number} format to query ref-log */
        reflog_len = at = 0;
diff --git a/t/t1512-rev-parse-disambiguation.sh 
b/t/t1512-rev-parse-disambiguation.sh
index 6b3d797..97ff8ac 100755
--- a/t/t1512-rev-parse-disambiguation.sh
+++ b/t/t1512-rev-parse-disambiguation.sh
@@ -261,4 +261,19 @@ test_expect_success 'rev-parse --disambiguate' '
        test "$(sed -e "s/^\(.........\).*/\1/" actual | sort -u)" = 000000000
 '
 
+test_expect_success 'rev-parse 20-hex ref' '
+       REF=`git rev-parse HEAD` &&
+       VAL=`echo| git commit-tree 4b825dc642cb6eb9a060e54bf8d69288fbee4904` &&
+       git update-ref refs/heads/$REF $VAL &&
+       test `git rev-parse $REF` = $VAL
+'
+
+test_expect_success 'rev-parse ambiguous 20-hex ref' '
+       REF=`git rev-parse HEAD` &&
+       VAL=`echo| git commit-tree -p HEAD 
4b825dc642cb6eb9a060e54bf8d69288fbee4904` &&
+       git update-ref refs/tags/$REF $VAL &&
+       test `git rev-parse $REF 2>err` = $VAL &&
+       grep "refname.*ambiguous" err
+'
+
 test_done
-- 
1.8.2.83.gc99314b

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