Generate diffs between base and tip commits

Review Request #10130 — Created Aug. 27, 2018 and submitted — Latest diff uploaded

Information

Review Board
release-4.0.x
45b9779...

Reviewers

The get_diff_files utility can now generate a list of files to diff
that exist between two commits in a diffset. This allows us to generate
diffs between two commits (and therefore to see the diff of any
individual commit). There is currently no UI support for this behaviour,
but that is comming in a future patch.

Ran unit tests.