Payload is the content of the last text part of the multipart message.

Mailing lists sometimes append their own footers as a new part. This
happens for emails that are already multipart or the ones with more
uncommon encodings.

In those cases we end up with an empty comment, as the footer is then
stripped out via clean_content().

We cannot use the commentbuf as it has anything diff-like stripped out
by parse_patch().

Fixes #39



Signed-off-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>