keyword: support rollback by restoring expansion to previous values Prevent spurious differences in the working directory after a rollback. Add tests for several rollback situations.

keyword: do not expand at all during diff Always shrink and never expand keywords during a diff operation. Avoid user distraction e.g. because of spurious differences appearing in the commit editor.

keyword: restore restricted read mode value after overwriting Even though just enforcing expansion after overwriting files in the working directory caused no problems that we know of, this avoids a potential source of problems (e.g. in collaboration other extensions) at no costs.