[PATCH] Make do_each_ref() follow symlinks.

[PATCH] Make do_each_ref() follow symlinks.

Because there is no reason not to, and this makes things a bit
safer when running "git prune".

[jc: I do not necessarily agree with the reasoning of the
original author that it is a good way to keep "git prune" from
stomping on objects to have a symlink that points at .git/refs
of the repository A in the .git/refs of the repository B when
repository A borrows object database from repository B.  For one
thing, the object database that everybody borrows objects from
may not even have its own .git/refs hierarchy.  Come to think of
it, maybe we should disallow symlink inside .git/refs hierarchy;
we update the files there by creat/rename pair, so having
symlinks would not work anyway when you do anything that would
update them.]

Signed-off-by: Junio C Hamano <junkio@cox.net>