Add 'path' to comment about composition of file id
Comment on each diff endpoint
Reduce duplication in file diff endpoint
Make decode_json_ok helper to DRY common idiom
Appease perl critic in t/ and remove from exclusions
Make diff tests easier to read and test more attributes
Test that each /diff endpoint can output raw text

refs #250.
Make raw diff available from all diff endpoints

Move common logic a single method
and just get the source/target in each endpoint.

fixes #250.
Appease perl-critic in recently edited files