Create a basic DiffCreatorTest

This does not yet exercise anything useful, but it's a good start for
writing a meaningful DiffCreatorTest.

#130 - Can not show diff and create review request
Rewrite the DiffCreator to use diff instead of patch

This allows us to easily control the patch header, including the Index,
added and removed line.

This does not yet handle renames, so it's not 100% done.