parsemail: ignore html part of multi-part comments

parsemail: ignore html part of multi-part comments

Currently an html-protection present only for patch-emails. If a
multi-part comment-email arrives, it messes up patchwork. In my case,
the symptom was a non intended 'Signed-off-by' in the downloaded
patches, with html-like junk.

This patch makes parsemail skip all parts of comment which are not
text/plain.

Of course, this will drop html-only emails completely. But they can
not be parsed anyways.

Signed-off-by: Yuri Volchkov <yuri.volchkov@gmail.com>
Reviewed-by: Stephen Finucane <stephen@that.guru>
(cherry picked from commit e27ff061dc01e51967a978884a5c59152863ab9c)