Perforce has some complex diff logic, and not all of it is instrumented

or even comprehensively documented based on actual usage.
In preparation for work on modernizing the diff building, this change

adds type annotations for all the _extract*() functions used to

calculate add/delete/edit/move information, and updates documentation

accordingly. To keep code readable, these functions now require keyword

arguments, and flags have defaults.
As part of this, mypy and pyright determined there were places where

None values could theoretically be passed in as paths or revisions in

some places. This would have led to broken diffs before, but shouldn't

really happen. We now assert these values as a precaution.
It was also found that revisions are sometimes passed as a string, and

sometimes as an integer. The functions themselves can handle either, but

we now formalize this in the type annotations. We may wish to

standardize it at a later date.

Unit tests pass.