In merge requests, add ability to view how discussed changes were applied

Problem to solve

Currently, if developers post comments/discussions in Merge Requests, it is hard for them to easily compare the patch before applying discussed changes, the discussion and the code after applying the changes. The side-by-side view shows 2 versions of the code, but not discussions - discussions dissappear if the code changes. Discussions show "@skurek changed this line in version 2 of the diff about 2 hours ago" - but this requires opening a new browser tab for every discussion.

Proposal

Allow merge requests discussions to be visible on the previous file versions, when comparing side-by-side. This should work nicely with gitlab's dropdowns allowing to choose "Changes between ... and ... ", and should greatly improve code review experience in Gitlab :)

As an example of this feature working, take a look at Android's gerrit-based code review system. See this example patchset. The discussion is visible on the left, while the change is visible on the right.

We are currently using GitLab CE 10.4.3. If this feature is available in Gitlab EE, or in a newer version of Gitlab, then please point me there and sorry for the trouble :)