transport: report missing submodule pushes consistently on stderr

The surrounding advice is printed to stderr, but the list of submodules
is not. Make the report consistent by reporting everything to stderr.
Signed-off-by: NStefan Beller <sbeller@google.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

transport.c

@@ -771,7 +771,7 @@ static void die_with_unpushed_submodules(struct string_list *needs_pushing)
 	fprintf(stderr, _("The following submodule paths contain changes that can\n"
 			"not be found on any remote:\n"));
 	for (i = 0; i < needs_pushing->nr; i++)
-		printf("  %s\n", needs_pushing->items[i].string);
+		fprintf(stderr, "  %s\n", needs_pushing->items[i].string);
 	fprintf(stderr, _("\nPlease try\n\n"
 			  "	git push --recurse-submodules=on-demand\n\n"
 			  "or cd to the path and use\n\n"