Add some more tests after #26

[kit-ty-kate]

I don't have the energy to make all the tests so i'll leave it for future me before the final 3.0.0 release to do it.
To test:

• exotic git file prefixes (not just a and b)
• isomorphicity of:
  • all 3 git extensions
  • all 3 git extensions with a normal git diff right after modifying it
  • all 3 git extensions with another unrelated file right after
• behaviour of the 2 git filename parsers when encountering spaces
• make sure the diff function works fine in presence of empty and non-existent files