Add the `github-link` git command.

This git command will open or output a link to GitHub based on the
current HEAD or a provided ref. You must include the `github.project`
value in your git config so we know where to point the URL towards.
Set `gc.autodetach` to `true` in `gitconfig`.
Include the author's email in `git-rank-contributors` when not using a mailmap.
Change the git aliases `origin-cherry` and `oc` to `ocherry`.
Add the `odiff` git alias.

This git alias diffs against the origin branch of the same name.