stgit: per branch cover letters

It would be convenient to have per branch cover letters that are stored by stg and get automatically applied when a patch set is mailed. Use 'stg edit' to alter them. The first time you email a series you would get an editor to write the cover letter if you hadn't already done it. You could track the "to=" this way also.

Another useful feature would be a config option that says something like "prompt for new version" when mailed. Now each time you mail the series it throws you into the cover page editor with an incremented patch version. You can just quit out of the editor if you don't want to increment the version.

I didn't know I wanted features like this until I made a patch that has gone through 25 versions.