Handle changed file permissions

[lolrepeatlol]

Previously, kmax's FileChangeType class did not account for file permission changes in diff files, which caused an AssertionError to appear in those cases. This pull request simply adds another type, PERMISSION_CHANGED. Logic elsewhere isn't changed in the code.

Important notes:

• The change assumes that permission changes "aren't interesting" and aren't worth setting configuration options for.
• I changed the FileChangeType class's tabs to spaces to align with modern Python code styling.
• The pull request has only been tested on the steps under #274. It is not tested any further, so feel free to request changes.