Merge request versions broken

Summary

In case a merge request has 2 versions, 'changes between' options will appear above the diffs for the merge request. In case there are no changes for some files between version 1 and version 2, the diff shown between version 2 and version 1 is incorrect (shows the full changes against the base).

Steps to reproduce

Create a merge request, then amend the original commit for the merge request to be updated with the latest changes.

Expected behavior

Diff between version 2 and version 1 should show incremental changes only (or if the files have not changed, nothing).

Actual behavior

Diff between version 2 and version 1 shows changes compared to base.

Relevant logs and/or screenshots

See corresponding screen captures below for all diffs between base, version7 and latest:

Output of checks

This issue is on GitLab.com as of today (version 8.12.4).