Accept repository name in query string

Looking at just one repository is boring. Let’s take the owner,
repository and optional commit hash from the query string, in the
following format:

* [`?` [_owner_ `/` _repo_ [`/commits/` [_id_]]]]
  List the commits starting from _id_ or `master` if unspecified. If
  _owner_ and _repo_ are omitted, default to this repository.
* `?` _owner_ `/` _repo_ `/commit/` [_id_]
  Show a single commit _id_ or the last commit on `master`.