When a comment containing a diff is sent to the mailing list is has the
diff extracted out and we save just the comment body above the patch.

Let's fix that by using payload instead of commentbuf. We still strip
signatures and mailing list footers there.

NOTE: we still use commentbuf for splitting an actual patch into the
diff and the patch's comment.

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