Identical trees should not trigger error in cg-diff

Hello!

cg-diff reports error when comparing identical trees. It's not as useless as it may seem. For example, I make local changes, e-mail them upstream and then get them back via cg-update with minor changes. I revert the minor differences between master and origin. Here's what I get when I want to make sure the trees are identical (their history is not):

$ cg-diff -r origin:master
cg-diff: trying to diff b00f462cee5b02455abdbb476fa7c9e94a9fcb4c against
itself

What's worse, cg-diff exists with exit status 1, indicating an error. I don't see any reason for this to be an error. No difference between branches is OK. This may be especially important when cg-diff is used non-interactively in a script.

@@ -167,7 +167,7 @@ fi
 id1=$(tree-id "$id1") || exit 1
 id2=$(tree-id "$id2") || exit 1
 
-[ "$id1" = "$id2" ] && die "trying to diff $id1 against itself"
+[ "$id1" = "$id2" ] && exit 0
 
 cat $filter | xargs git-diff-tree -r -p $id1 $id2 | colorize | pager
 

Regards,
Pavel Roskin