Patches sent as attachments aren't well-formatted for 'pwc git-am' or 'git pw apply'

Example: http://patchwork.freedesktop.org/patch/68368/mbox/

Feeding that into `git pw apply` for xserver at commit `f3593918a0507cc080875788b2776ebe4f57c694` will create a commit whose topic is the containing Subject line, and whose body is a concatation of the containing message body and the attachment. It's possible to recover from this with clever use of `git commit --amend` but it'd be nice to get right up front.