resolve_ref_unsafe(): ensure flags is always set

If the caller passes flags==NULL, then set it to point at a local
scratch variable. This removes the need for a lot of "if (flags)"
guards in resolve_ref_1() and resolve_missing_loose_ref().

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