Support inter-"working copy" diffs

[ColMelvin]

Support diffs where the <since> commit is not in the SVN repository. The availability of these diffs should be limited, to avoid creating patches that cannot be applied to the database.

Add a flag that supports the following options:

1. Disallow inter-"working copy" diffs (default)
2. Allow inter-"working copy" diffs iff the resulting patch can be applied to the BASE revision
3. Allow inter-"working copy" diffs regardless