[perl #122870] Fix B::Deparse–DDS compatibility
Correct and update sv.c’s string copy comments

Thanks to Dave Mitchell for pointing out my mistakes.

Whether the rhs is copy-on-write already does not change whether we
do the long-string-and-left-buffer-size check.  The comments (added in
2ac0bcb35) were wrong.

Also, update the comments for e8c6a474e, which added the
big-buffer-on-the-right skip logic.
Sort MANIFEST properly
Maintainers.pl: Include all Deparse-*.t files in _PERLLIB