Stop using find in deep-cp, deep-rm, etc.

These functions were written using `find` to avoid globbing with `.*` which
could accidentally traverse to the parent directory.  Since `extglob` is
already enabled, we can just use an extended glob to accomplish the same.

This pull request also fixes a trivial issue where `cp` could fail if nothing
matches the glob `!(tmp)`.

There's nothing urgent about this pull request, but it seems like something
nice to clean up the next time you're doing a round of testing anyway. :-)