Add a test for J (join lines).
Add bracketing of strings (and other) we print out in assertEq failures.
Fix for J to make it pass the test - do our own joining rather than using
buffer.join_lines.
Add a test for 'de'.
Fix the code: allow for both inclusive and exclusive motions as well as
linewise, and adjust all uses to use the type rather than true/false for
the formerly-named linewise parameter to do_movement.