The treeish disambiguation function tries to peel tags, but
it does so by calling:

  deref_tag(lookup_object(sha1), ...);

This will only work if we have previously looked at the tag
and created a "struct tag" for it. Since parsing revision
arguments typically happens before anything else, this is
usually not the case, and we would fail to peel the tag (we
are lucky that deref_tag() gracefully handles the NULL and
does not segfault).

Instead, we can use parse_object(). Note that this is the
same fix done by 94d75d1 (get_short_sha1(): correctly
disambiguate type-limited abbreviation, 2013-07-01), but
that commit fixed only the committish disambiguator, and
left the bug in the treeish one.

Signed-off-by: Jeff King <p...@peff.net>
---
 sha1_name.c                         | 2 +-
 t/t1512-rev-parse-disambiguation.sh | 7 +++++++
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/sha1_name.c b/sha1_name.c
index 38e51d9..432a308 100644
--- a/sha1_name.c
+++ b/sha1_name.c
@@ -269,7 +269,7 @@ static int disambiguate_treeish_only(const unsigned char 
*sha1, void *cb_data_un
                return 0;
 
        /* We need to do this the hard way... */
-       obj = deref_tag(lookup_object(sha1), NULL, 0);
+       obj = deref_tag(parse_object(sha1), NULL, 0);
        if (obj && (obj->type == OBJ_TREE || obj->type == OBJ_COMMIT))
                return 1;
        return 0;
diff --git a/t/t1512-rev-parse-disambiguation.sh 
b/t/t1512-rev-parse-disambiguation.sh
index 30e0b80..dfd3567 100755
--- a/t/t1512-rev-parse-disambiguation.sh
+++ b/t/t1512-rev-parse-disambiguation.sh
@@ -264,6 +264,13 @@ test_expect_success 'ambiguous commit-ish' '
        test_must_fail git log 000000000...
 '
 
+# There are three objects with this prefix: a blob, a tree, and a tag. We know
+# the blob will not pass as a treeish, but the tree and tag should (and thus
+# cause an error).
+test_expect_success 'ambiguous tags peel to treeish' '
+       test_must_fail git rev-parse 0000000000f^{tree}
+'
+
 test_expect_success 'rev-parse --disambiguate' '
        # The test creates 16 objects that share the prefix and two
        # commits created by commit-tree in earlier tests share a
-- 
2.10.0.492.g14f803f

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