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 two static items at the beginning and end of the commit list
  and style them 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 URLs and
  the page numbers from them; set suitable `href`s on the `next` and
  `prev` links so that the user can bookmark those or open in another tab.
* At the same time, hook an `onclick` function on that same link, which
  will fetch the next or previous page without reloading. Note that we
  have to pass a reference position where the newly fetched commits need
  to be inserted. We pass the element _after_ which new items go, since
  we want them to end up in reverse direction.