To ensure that copied refs can always be freed w/o causing a
double-free, make copy_ref() perform a deep copy.

Also have copy_ref() return NULL if asked to copy NULL to simplify
things for the caller.

Background: currently copy_ref() performs a shallow copy. This is fine
for current callers who never free the result and/or only copy refs
which contain NULL pointers. But copy_ref() is about to gain a new
caller (guess_remote_head()) which copies refs where peer_ref is not
NULL and the caller of guess_remote_head() will want to free the result.
Signed-off-by: NJay Soffian <jaysoffian@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>