The documentation is not complete yet, and I haven't tested it yet
extensively, but it fundamentally does the right thing.  I am not too
sure about the generic interface and the plist that `prepare-patch'
returns.  There might be a better way to do this, generating less
garbage, but I'm stuck between trying to be VC-generic and MUA-generic
which is unsurprisingly tricky.

> Once the file opens in message-mode, most likely we need to strip the
> magic From <sha> <date> header from the beginning of the mail. Then we ensure
> don't do any nasty whitespace removal or wrapping.
>
> Most likely, depending on the backend, we should not require any
> parameters besides the "set of changes". For instance, in Git you can
> configure `git-format-patch` in the git configuration for several
> attributes, like --to=, --annotate, --prefix, etc.
>
> I don't remember how Mercurial works with this. Probably similar. It
> should generate mbox entries as well, I think.