refs: implement iteration over only per-worktree refs

refs: implement iteration over only per-worktree refs

Alternate refs backends might still use files to store per-worktree
refs. So provide a way to iterate over only the per-worktree references
in a ref_store. The other backend can set up a files ref_store and
iterate using the new DO_FOR_EACH_PER_WORKTREE_ONLY flag when iterating.

Signed-off-by: David Turner <dturner@twopensource.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>