remote rename: warn when refspec was not updated

remote rename: warn when refspec was not updated

When renaming a remote, we also try to update the fetch refspec
accordingly, but only if it has the default format. For others, such
as refs/heads/master:refs/heads/origin, we are conservative and leave
it untouched. Let's give the user a warning about refspecs that are
not updated, so he can manually update the config if necessary.

Suggested-by: Jeff King <peff@peff.net>
Signed-off-by: Martin von Zweigbergk <martin.von.zweigbergk@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>