Added Diff action.
TODO comment
Rename CommitIterator -> CommitWalker
Remove unneeded BUILDARGS.
clear_cache() didn't actually remove the index files.  Now it does.
Added Patch class, for applying a Diff to a stack.
Holy Cow!  The Revert action works again.
Forgot to update calls to Pinto::Commit->new, which now required named args.