Previously in history views, revisions were ordered according to the
BFS traversal of the revisions DAG.
This commit adds three new ordering type modifying the history display:
* Committer date ordering: revisions are displayed in reverse chronological order
of their commit date
* DFS: revisions are ordered according to the DFS traversal of the revisions DAG
* DFS post: revisions are ordered according to the DFS post traversal of the
revisions DAG
The four types of revisions ordering can be easily switched through the UI.
An implementation of a filtered history, showing only revisions that modify a
specific path, is also introduced. Nevertheless, it is not used in swh-web/browse
due to performance issues when walking across a really large history.
Closes T1026
Related T1284