Add pagination

Up to now, the commit list only shows the last 30 commits. This might be
enough for a small repository like this one, but not for real
repositories.

When there are more commits in the selected branch, GitHub sends a
`Link:` header in the response. We’re supposed to use it to fetch more
commits.

* We add a static item to the commit list and style it to be hidden at
  first.
* We add an optional `/pages/` _page_ part to our query string. If
  specified, we add the `&page=` _page_ parameter to the API query.
* If the API responds with a `Link:`, parse out the actual link URL and
  the page number from it; set a suitable `href` on the `more` link so
  that the user can bookmark that or open in another tab.
* At the same time, hook an `onclick` function on that same link, which
  will fetch the next page without reloading.

Unfortunately, GitHub only gives the “next page” link, not for
previous/first/last pages. So we cannot display a “more commits” link in
the other direction. (Well, actually we can, since we know the page
number we started at… will have to wait until later.)