Merge branch 'rh/peeling-tag-to-tag'

Make "foo^{tag}" to peel a tag to itself, i.e. no-op., and fail if
"foo" is not a tag.  "git rev-parse --verify v1.0^{tag}" would be a
more convenient way to say "test $(git cat-file -t v1.0) = tag".

* rh/peeling-tag-to-tag:
  peel_onion: do not assume length of x_type globals
  peel_onion(): add support for <rev>^{tag}

Documentation/revisions.txt

@@ -125,6 +125,9 @@ some output processing may assume ref names in UTF-8.
 object that exists, without requiring 'rev' to be a tag, and
 without dereferencing 'rev'; because a tag is already an object,
 it does not have to be dereferenced even once to get to an object.
++
+'rev{caret}\{tag\}' can be used to ensure that 'rev' identifies an
+existing tag object.
 
 '<rev>{caret}\{\}', e.g. 'v0.99.8{caret}\{\}'::
   A suffix '{caret}' followed by an empty brace pair

sha1_name.c

@@ -677,11 +677,13 @@ static int peel_onion(const char *name, int len, unsigned char *sha1)
 		return -1;
 
 	sp++; /* beginning of type name, or closing brace for empty */
-	if (!strncmp(commit_type, sp, 6) && sp[6] == '}')
+	if (!prefixcmp(sp, "commit}"))
 		expected_type = OBJ_COMMIT;
-	else if (!strncmp(tree_type, sp, 4) && sp[4] == '}')
+	else if (!prefixcmp(sp, "tag}"))
+		expected_type = OBJ_TAG;
+	else if (!prefixcmp(sp, "tree}"))
 		expected_type = OBJ_TREE;
-	else if (!strncmp(blob_type, sp, 4) && sp[4] == '}')
+	else if (!prefixcmp(sp, "blob}"))
 		expected_type = OBJ_BLOB;
 	else if (!prefixcmp(sp, "object}"))
 		expected_type = OBJ_ANY;

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 &&