patch.diffopts: add support for noprefix In an upcoming patch we'll enable support as an option to 'hg diff' as well. The tests reflect the current state of the world -- as we add support we'll see changes in the test output.

patch.diffopts: allow a setting to be forced in plain mode Upcoming patches will add an option that will almost certainly break diff output parsers when enabled. Add support for forcing an option to something in plain mode, as a fallback. Options passed in via the CLI are not affected, though -- it is assumed that any script passing the option in explicitly knows what it is doing.