Fixes for packaging

- Shorten package description line
- Fix formatting of trailing line
- Add autoload cookie

This is in connection with https://github.com/milkypostman/melpa/pull/1510.

I'd also suggest doing most of the work of the `git-link` function in a non-interactive function which could be called programmatically from elsewhere without messing up the kill ring -- this is potentially a widely useful utility function. :-)