Trivial cleanup of track_tree_refs()

This makes "track_tree_refs()" use the same "tree_entry()" function for
counting the entries as it does for actually traversing them a few lines
later.

Not a biggie, but the reason I care was that this was the only user of
"update_tree_entry()" that didn't actually *extract* the tree entry first.
It doesn't matter as things stand now, but it meant that a separate
test-patch I had that avoided a few more "strlen()" calls by just saving
the entry length in the entry descriptor and using it directly when
updating wouldn't work without this patch.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

tree.c

@@ -153,10 +153,8 @@ static void track_tree_refs(struct tree *item)
 	/* Count how many entries there are.. */
 	desc.buf = item->buffer;
 	desc.size = item->size;
-	while (desc.size) {
+	while (tree_entry(&desc, &entry))
 		n_refs++;
-		update_tree_entry(&desc);
-	}
 
 	/* Allocate object refs and walk it again.. */
 	i = 0;