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/fast-export.c

@@ -963,7 +963,7 @@ static void import_marks(char *input_file)
 			/* only commits */
 			continue;
 
-		commit = lookup_commit(&oid);
+		commit = lookup_commit(the_repository, &oid);
 		if (!commit)
 			die("not a commit? can't happen: %s", oid_to_hex(&oid));