Consolidate ignore_packed logic more

This refactors three loops that check if a given packfile is on the
ignore_packed list into a function is_kept_pack().  The function returns
false for a pack on the list, and true for a pack not on the list, because
this list is solely used by "git repack" to pass list of packfiles that do
not have corresponding .keep files, i.e. a packfile not on the list is
"kept".

Signed-off-by: Junio C Hamano <gitster@pobox.com>

revision.h

@@ -159,5 +159,6 @@ enum commit_action {
 extern enum commit_action simplify_commit(struct rev_info *revs, struct commit *commit);
 
 extern int has_sha1_kept_pack(const unsigned char *sha1, const struct rev_info *);
+extern int is_kept_pack(const struct packed_git *, const struct rev_info *);
 
 #endif