peel_onion(): add support for <rev>^{tag}

Complete the <rev>^{<type>} family of object descriptors by having
<rev>^{tag} dereference <rev> until a tag object is found (or fail if
unable).

At first glance this may not seem very useful, as commits, trees, and
blobs cannot be peeled to a tag, and a tag would just peel to itself.
However, this can be used to ensure that <rev> names a tag object:

    $ git rev-parse --verify v1.8.4^{tag}
    04f013dc38d7512eadb915eba22efc414f18b869
    $ git rev-parse --verify master^{tag}
    error: master^{tag}: expected tag type, but the object dereferences to tree type
    fatal: Needed a single revision

Users can already ensure that <rev> is a tag object by checking the
output of 'git cat-file -t <rev>', but:
  * users may expect <rev>^{tag} to exist given that <rev>^{commit},
    <rev>^{tree}, and <rev>^{blob} all exist
  * this syntax is more convenient/natural in some circumstances

Signed-off-by: Richard Hansen <rhansen@bbn.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 7 insertions, 0 deletions

diff --git a/t/t1511-rev-parse-caret.sh b/t/t1511-rev-parse-caret.sh
index eaefc777bd..15973f2094 100755
--- a/t/t1511-rev-parse-caret.sh
+++ b/t/t1511-rev-parse-caret.sh
@@ -54,6 +54,13 @@ test_expect_success 'ref^{tree}' '
 	test_must_fail git rev-parse blob-tag^{tree}
 '
 
+test_expect_success 'ref^{tag}' '
+	test_must_fail git rev-parse HEAD^{tag} &&
+	git rev-parse commit-tag >expected &&
+	git rev-parse commit-tag^{tag} >actual &&
+	test_cmp expected actual
+'
+
 test_expect_success 'ref^{/.}' '
 	git rev-parse master >expected &&
 	git rev-parse master^{/.} >actual &&