refs: refactor resolve_gitlink_ref() to accept a repository

In `resolve_gitlink_ref()` we implicitly rely on `the_repository` to
look up the submodule ref store. Now that we can look up submodule ref
stores for arbitrary repositories we can improve this function to
instead accept a repository as parameter for which we want to resolve
the gitlink.

Do so and adjust callers accordingly.

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

object-file.c

@@ -2669,7 +2669,7 @@ int index_path(struct index_state *istate, struct object_id *oid,
 		strbuf_release(&sb);
 		break;
 	case S_IFDIR:
-		return resolve_gitlink_ref(path, "HEAD", oid);
+		return repo_resolve_gitlink_ref(the_repository, path, "HEAD", oid);
 	default:
 		return error(_("%s: unsupported file type"), path);
 	}