Merge branch 'change-diff-and-patch-specs' into 'master'

Change the revspec for `git format-patch`

For merge requests, GitLab currently sends:

* ShaFrom == tip of target branch
* ShaTo == tip of source branch

This MR adds tests for this scenario and the alternative:

* ShaFrom == base SHA
* ShaTo == tip of source branch

It then makes the test for the first case pass by altering the revspec. 

What to do with diffs is still an open question, so we leave those untouched for the moment.

See https://gitlab.com/gitlab-org/gitlab-ce/issues/22229

See merge request !68