This fleshes out the remainder of reviewboard.diffviewer.commit_utils

with typing and documentation. The validation payloads are now fully

typed, which is only useful internally as this is considered opaque

outside this module.
There is one fix with equality checking for CommitHistoryDiffEntry. It

assumed the other type, and would crash if given a different type. We

now check for this.
This is mostly the same as Christian's change at /r/14416. The changes

I've made:

Moved SerializedCommitHistoryDiffEntryType so it's defined before

  it's used.
Fixed a typo in the DiffCommitsValidationInfo docs.
Fixed up some Args/Returns types to list the specific

  TypedDict/TypeAlias names instead of just str or dict.

Ran unit tests.