While investigating issues reported by a customer around out-of-order

FileDiffs, I added some typing in order to ensure that there weren't any

surprises in the code or the way that data was structured. In the

process, a few small documentation issues were found, along with a

couple of theoretical type-safety issues.
This change adds type hints throughout this code and ensures safe usage

of values that could be None. It also switches to keyword-only

arguments to allow for future expansion without risk of surprises.
It does not otherwise change anything about FileDiff creation.

Unit tests pass.