github-commits: Add HUBOT_GITIO and HUBOT_COMMIT_ONELINE

Details and motivation in the commit messages.
On Sat, May 24, 2014 at 07:51:09AM -0700, Josh Nichols wrote:
> To get around that, I'd suggest refactoring most of the logic to
> it's own function, …

I've pulled this out into githubot with iangreenleaf/githubot#25.
Once that lands and is released, I'll update this PR with the
associated githubot bumps and API changes.