SubmittingPatches: clarify GitHub visual

SubmittingPatches: clarify GitHub visual

GitHub has two general forms for its states, sometimes they're a simple
colored object (e.g. green check or red x), and sometimes there's also a
colored container (e.g. green box or red circle) which contains that
object (e.g. check or x).

That's a lot of words to try to describe things, but in general, the key
for a failure is that it's recognized as an `x` and that it's associated
with the color red -- the color of course is problematic for people who
are red-green color-blind, but that's why they are paired with distinct
shapes.

Signed-off-by: Josh Soref <jsoref@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>