Make merge-ref diffs the default comparison mode

Problem to solve

Merge request diffs are currently calculated by git diff target...source which compares HEAD of target with the merge base of target and source. This works well until changes from the target branch are merged in to the source branch, creating a complete mess of the diff.

Further details

In previous iterations we've added a new diff mode, and implemented commenting support. This issue will make it the default comparison mode.

Proposal

This should definitely be feature flagged, so that it can be enabled for individual projects and groups for testing.

When viewing a merge request diff, when comparing between Latest version and master this should default to comparing using the merge-ref. The merge-base method should not be available any more.

Links / references