Refactor parse_diffoscope_json

[sei-aderr]

Refactor of parse_diffoscope_json function. Mainly followed the suggestions from Sebastian from a while ago that I saved in the issue

• Made the functionality be contained within a class
• Made a _check_flags function, and made each separate check into its own function
• Made the recursion a separate function
• Moved all handling of recursive iteration into the recursion function
• Renamed _process_and_save_results to _process_and_write_results
• Renamed is_path to is_abs_path and added tests
• Removed no longer used parent_source parameters and class variables from Diff

Would appreciate a quick review of how it looks now, to see if there are other changes that should be made before I write unit tests for each sub function if they were to change.

[sei-secheverria]

Very good improvements, several comments below

[sei-aderr]

Consider renaming plus_line and minus_line variables and any other references to them.

[sei-aderr]

Ready for another review. Lihan fixed the other tests in test_diff_command which will be pushed shortly. Once these pieces look good I will flesh out the unit tests for the new code.

[sei-lzhan]

diff_command unit test fixed, ready for review

[sei-secheverria]

Looks good, a couple of very minor points

[sei-kpitstick]

Tested, results line up.