GitLab has been slowly decommissioning old API endpoints, especially on

the SaaS gitlab.org. They recently got rid of the old diff endpoint,

which we were still using because the v4 API doesn't have a great way to

get the diffs. Unfortunately, this means we need to actually implement

support for v4.
This works somewhat similarly to how the GitHub diff API works, where

instead of just getting a full copy of the diff, we get a JSON blob that

includes diff fragments for each file. This is somewhat complicated by

the fact that said diff fragments do not contain the blob SHAs for the

old and new revisions. We have to go get that information from the file

API, which is a bit chatty but at least we can just use a HEAD request

to get only the metadata instead of having to fetch the file content.

Added a bunch of GitLab repositories and created post-commit review

  requests for commits that included modified, added, and deleted files.

  Saw things work every time.
Ran unit tests.