[PATCH] Dereference tag repeatedly until we get a non-tag.

When we allow a tag object in place of a commit object, we only
dereferenced the given tag once, which causes a tag that points
at a tag that points at a commit to be rejected.  Instead,
dereference tag repeatedly until we get a non-tag.

This patch makes change to two functions:

 - commit.c::lookup_commit_reference() is used by merge-base,
   rev-tree and rev-parse to convert user supplied SHA1 to that of
   a commit.
 - rev-list uses its own get_commit_reference() to do the same.

Dereferencing tags this way helps both of these uses.

Signed-off-by: Junio C Hamano <redacted>
---

*** Whether having a tag pointing at another tag is a separate
*** issue, but I do not see a reason to forbid it.  Maybe it
*** is used to represent a chain of trust.

 commit.c   |    5 +++--
 rev-list.c |    4 ++--
 2 files changed, 5 insertions(+), 4 deletions(-)

0dc9377363ee73c5e3f3711d6f82e49886ce8c6a

diff --git a/commit.c b/commit.c
--- a/commit.c
+++ b/commit.c

@@ -52,8 +52,9 @@ struct commit *lookup_commit_reference(c
 
 	if (!obj)
 		return NULL;
-	if (obj->type == tag_type)
-		obj = ((struct tag *)obj)->tagged;
+	while (obj->type == tag_type)
+		obj = parse_object(((struct tag *)obj)->tagged->sha1);
+
 	return check_commit(obj, sha1);
 }
 

diff --git a/rev-list.c b/rev-list.c
--- a/rev-list.c
+++ b/rev-list.c

@@ -367,12 +367,12 @@ static struct commit *get_commit_referen
 	/*
 	 * Tag object? Look what it points to..
 	 */
-	if (object->type == tag_type) {
+	while (object->type == tag_type) {
 		struct tag *tag = (struct tag *) object;
 		object->flags |= flags;
 		if (tag_objects && !(object->flags & UNINTERESTING))
 			add_pending_object(object, tag->tag);
-		object = tag->tagged;
+		object = parse_object(tag->tagged->sha1);
 	}
 
 	/*