patchtest-send-results: use Message-ID directly

patchtest-send-results: use Message-ID directly

There's no need to use regex for extracting the Message-ID field from
the patch email and mangle it by removing the angle brackets in the
process. Pull it directly from the mbox so that Patchtest's replies have
even fewer differences when compared to other replies. Also add a TODO
so that it's clear this needs adjustment when full series support is
added.

Signed-off-by: Trevor Gamblin <tgamblin@baylibre.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>