Bump Octokit to a more modern version.

This broke one test, which was expecting `per_page` and `sha` parameters
in the GitHub API call; Octokit now appears to defer to the API's
defaults.
Show the list of unreleased commits, if available.

Instead of just displaying the last page of commits from the GitHub API,
we can use the comparison API to get all unreleased commits. This is
useful for apps that tend to end up with large pull requests, or which
end up getting batches of features released at once; in either case, the
latest released commit may not be visible in the commit log.
Show undeployed commits in reverse order.

There's a fair amount of slightly hacky testing code in here; I'm not
too worried about that, because I'm planning to pull out all the commit
history code into the model layer shortly.
Include the latest release in the commit log.

This should help anchor the list and provide some reassurance that we
haven't missed out any commits.
Merge pull request #59 from alphagov/show-all-unreleased-commits

Show all unreleased commits