shallow: add repository argument to is_repository_shallow

Add a repository argument to allow callers of is_repository_shallow
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/rev-parse.c

@@ -879,7 +879,8 @@ int cmd_rev_parse(int argc, const char **argv, const char *prefix)
 				continue;
 			}
 			if (!strcmp(arg, "--is-shallow-repository")) {
-				printf("%s\n", is_repository_shallow() ? "true"
+				printf("%s\n",
+						is_repository_shallow(the_repository) ? "true"
 						: "false");
 				continue;
 			}