browse: Add new orderings for revisions history views

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.

Closes T1026