Add controls to select base and tip commits in the RB.DiffCommitListView

Review Request #1429 — Created Oct. 25, 2018 and updated

Review Board
The `RB.DiffCommitListView` can now optionally display controls for
selecting between base and tip commits to control what will be displayed
in the diffviewer. The actual logic for changing the diffs is not yet
implemented -- just the control itself.

Testing Done:
Ran JS tests.

Reviewed at

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Description From Last Updated
tewst~ ?? guest6459 guest6459