uploader: Use text before the first diff line as a description

[sjl]

Imagine uploading a patch that looks like this:

This patch does X.

diff a/hgext/mq.py b/hgext/mq.py
--- a/hgext/mq.py
+++ b/hgext/mq.py

It would be great if the uploader script would look for the first diff line and treat everything before that as the description. This would be especially nice because you could run hg export . | patchbin and it would upload the patch while using the commit message and other metadata as the description.

I've been working on patch-parsing in Mercurial's MQ extension over the past week or so, so I know exactly how this can be done. I'll add it this weekend.