[PATCH 2/2] lookup_commit_reference_gently: do not read non-{tag,commit}

lookup_commit_reference_gently unconditionally parses the object given
to it.  This slows down git-describe a lot if you have a repository
with large tagged blobs in it: parse_object() will read the entire
blob and verify that its sha1 matches, only to then throw it away.

Speed it up by checking the type with sha1_object_info() prior to
unpacking.

The reason that deref_tag() does not need the same fix is a bit
subtle: parse_tag_buffer() does not fill the 'tagged' member of the
tag struct if the tagged object is a blob.

Reported-by: Alex Bennée <redacted>
Signed-off-by: Thomas Rast <redacted>
---
 commit.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/commit.c b/commit.c
index 888e02a..00e8d4a 100644
--- a/commit.c
+++ b/commit.c

@@ -31,8 +31,12 @@ static struct commit *check_commit(struct object *obj,
 struct commit *lookup_commit_reference_gently(const unsigned char *sha1,
 					      int quiet)
 {
-	struct object *obj = deref_tag(parse_object(sha1), NULL, 0);
-
+	struct object *obj;
+	int type = sha1_object_info(sha1, NULL);
+	/* If it's neither tag nor commit, parsing the object is wasted effort */
+	if (type != OBJ_TAG && type != OBJ_COMMIT)
+		return NULL;
+	obj = deref_tag(parse_object(sha1), NULL, 0);
 	if (!obj)
 		return NULL;
 	return check_commit(obj, sha1, quiet);

-- 
1.8.3.506.g4fdeee5