refs: prepare refs_init_db() for initializing worktree refs

The purpose of `refs_init_db()` is to initialize the on-disk files of a
new ref database. The function is quite inflexible right now though, as
callers can neither specify the `struct ref_store` nor can they pass any
flags.

Refactor the interface to accept both of these. This will be required so
that we can start initializing per-worktree ref databases via the ref
backend instead of open-coding the initialization in "worktree.c".

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs/packed-backend.c

@@ -1246,6 +1246,7 @@ static const char PACKED_REFS_HEADER[] =
 	"# pack-refs with: peeled fully-peeled sorted \n";
 
 static int packed_init_db(struct ref_store *ref_store UNUSED,
+			  int flags UNUSED,
 			  struct strbuf *err UNUSED)
 {
 	/* Nothing to do. */