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>

tree.c

@@ -101,7 +101,7 @@ static int read_tree_1(struct tree *tree, struct strbuf *base,
 		else if (S_ISGITLINK(entry.mode)) {
 			struct commit *commit;
 
-			commit = lookup_commit(entry.oid);
+			commit = lookup_commit(the_repository, entry.oid);
 			if (!commit)
 				die("Commit %s in submodule path %s%s not found",
 				    oid_to_hex(entry.oid),