commit: add repository argument to lookup_commit

Add a repository argument to allow callers of lookup_commit to be more
specific about which repository to handle. This is a small mechanical
change; it doesn't change the implementation to handle repositories
other than the_repository yet.

As with the previous commits, use a macro to catch callers passing a
repository other than the_repository at compile time.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/am.c

@@ -1633,7 +1633,8 @@ static void do_commit(const struct am_state *state)
 
 	if (!get_oid_commit("HEAD", &parent)) {
 		old_oid = &parent;
-		commit_list_insert(lookup_commit(&parent), &parents);
+		commit_list_insert(lookup_commit(the_repository, &parent),
+				   &parents);
 	} else {
 		old_oid = NULL;
 		say(state, stderr, _("applying to an empty history"));