Add convenience method for getting a diff of a stack.

By default, the diff is against the parent.
Rename Config action -> Props.

To disambiguate from the repo Config class.
Allow for British spelling.
Redesigned the Differences.
Better variable names.
Convenience method for getting a Diff on a revision.
Just commentary and POD edits.
Remove trailing whitespace.
Make the Diff action use new Differences API.
Redesigned workflow for generating commit messages.
Use the polymorphic Difference constructor.

Also delay loading until we really need it.
Experimenting with different presentations for registration flags.
Use polymorphic Difference constructor.
Rename Reindex, Pop -> Register, Unregister.

I'll use Reindex to mean a "real" reindexing.
The left stack defaults to the default stack if not specified.