submodule--helper: prefer strip_suffix() to ends_with()

submodule--helper: prefer strip_suffix() to ends_with()

Using strip_suffix() lets us avoid repeating ourselves. It also makes
the handling of "/" a bit less subtle (we strip one less character than
we matched in order to leave it in place, but we can just as easily
include the "/" when we add more path components).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>