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.
Depends on D629