[PATCH] have commit-id dereference git tags

[Alexander Beyn <malex-git@fatelectrons.org>]

With this patch, if commit-id is passed a git tag, it will check to see
if the tag points to a commit. If it does, commit-id will return that
commit's id.

I'm not sure if this is the proper place to handle git tags, but it does
make things like "cg-diff -r v2.6.12-rc2:v2.6.12-rc3" work in the Linus
kernel tree. I've not noticed any problems this introduces, but I didn't 
do a thorough check.

Signed-Off-By: Alexander Beyn <malex-git@fatelectrons.org>


Index: commit-id
===================================================================
--- 6a20fd05c468097d5a5a47cd37b41581c963cf63/commit-id  (mode:100755 sha1:daf0ea1e35f1bee22eb4c4771495b2212d15f4e7)
+++ 66fded2e9d931ecc8b03aa282a7eb9559955c172/commit-id  (mode:100755 sha1:978c3ffeb072422436af7af1ad4a8de0e91c9632)
@@ -32,7 +32,15 @@
 	exit 1
 fi
 
-if [ "$(git-cat-file -t "$id")" != "commit" ]; then
+type="$(git-cat-file -t "$id")"
+
+if [ "$type" = "tag" ]; then
+	id=$(git-cat-file tag "$id" | egrep -m 1 "^object $SHA1")
+	id="${id#object }"
+	type="$(git-cat-file -t "$id")"
+fi
+
+if [ "$type" != "commit" ]; then
 	echo "Invalid id: $id" >&2
 	exit 1
 fi