github-commit-link: Add support for repo@<SHA> and user/repo@<SHA>

Based on similar parsing in github-issue-link.coffee.  The main change
in the logic is that the initial .* is now the non-greedy .*?, so it
doesn't gobble up the repo-part of the regexp [1].  The user/repo
portion is [^\s@]+, which matches one or more characters that are
neither whitespace [2] nor @.

I also document the fact that lines matching 'commit/' are ignored.

[1]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions#special-questionmark
[2]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions#special-white-space
github-commit-link: Only show the first line of the commit

This is traditionally the message summary.  From Git's own docs [1]:

> The first line of the commit message should be a short description
> (50 characters is the soft limit, see DISCUSSION in git-commit(1)),
> and should skip the full stop.

I sometimes write long, detailed commit messages (like this one), and
there's no need to spam the channel with all of this motivational
background.  The summary line should be sufficient context there, and
folks looking for more information can follow the URL (which I've
moved to it's own line).

[1]: https://github.com/git/git/blob/v1.9.0/Documentation/SubmittingPatches#L89