Implement Git::Log#follow

This implements Git::Log#follow, mentioned in #82.

In contrast to the issue description, it implements the follow option as a seperate method instead of a parameter.

The implementation doesn't check for sanity (`--follow` is only allowed for single file logs).

Example:
```
@repo.log.path(file_path).follow
@repo.log.path(file_path).follow(true).follow(false) #off again
```

Tests are in a seperate commit. There is now indication how manipulating the test suite git repo should be done, I just went ahead with what I think how it works. Feel free to remove the commit.