* Only operate on `/changeset`, not on `/diff` anymore
* Made first changeset argument optional. Therefore, a request
to `/changeset` will go the HEAD revision.
* The first changeset argument can now be anything upto the next "/".
This allows for URLs like `/changeset/HEAD` or `/changeset/latest`,
as well as being more friendly to other VersioningSystemBackend
(e.g. `/changeset/tip` or `/changeset/0d36e3d7e2ea`...)