refs: add methods to init refs db

Alternate refs backends might not need the refs/heads directory and so
on, so we make ref db initialization part of the backend.

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>

refs.c

@@ -1292,6 +1292,14 @@ static const char *resolve_ref_recursively(struct ref_store *refs,
 	return NULL;
 }
 
+/* backend functions */
+int refs_init_db(struct strbuf *err)
+{
+	struct ref_store *refs = get_ref_store(NULL);
+
+	return refs->be->init_db(refs, err);
+}
+
 const char *resolve_ref_unsafe(const char *refname, int resolve_flags,
 			       unsigned char *sha1, int *flags)
 {