Remove "DiffusionHistoryTableView" and "DiffusionHistoryView"

Summary:
Ref T13552.

Currently, the "Browse" page shows a snippet of unmerged changes if you're looking at a non-default branch. Remove this for consistency with the simplified main "Browse" page. This is reachable via "Compare".

Update the "Compare" page to use the new "CommitGraphView".

Test Plan:

• Looked at the "Browse" page of "stable".
• Looked at the "Compare" page for "stable vs master".

Maniphest Tasks: T13552

Differential Revision: https://secure.phabricator.com/D21414