Add diff commit selection UI to diffviewer

Review Request #7323 — Created May 20, 2015 and submitted — Latest diff uploaded


Review Board


The diffviewer can now select two commits in a commit history to
generate a diff between. This currently does not support inter-commit
diffs and the UI is disabled in that case.

Manually tested commit generation between individual commits and for
spans of commits.

Manually verified the commit selection UI is not shown for interdiffs.