Trivial cleanup of track_tree_refs()

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>