Mock git behaviour in unit testing

[TTitcombe]

Functionality of doculog relies on git. This makes it difficult to unit test because git cannot, as far as I know, easily have its behaviour altered/mocked for unit tests. It is possible to mock the responses to subprocess calls (which invoke git), but this requires the test writer to create a string of git log/diff output and isn't scalable.

We need to find a way to mock git behaviour for unit testing. I.e. start a git repo local to that test / test suite, easily add commits and tags.

[Seniatical]

command git init allows you to have an "empty" repo which you can commit to and do your other git stuff perfectly fine

[Seniatical]

you can add commits to these as well if I'm not wrong