Ad debug variable, fix subcommand state dir check

`cppr` was checking `$state_dir/pcp_state` and `$state_dir/ppr_state`
instead of `$GIT_DIR/xyz` to determine if a `pcp` or `ppr` run was still in
progress

Setting `DEBUG_CPPR` to a non-blank string will run all these scripts
with the `-x` option set.

`git cppr --continue` now runs `git pcp --continue` or `git ppr
--continue` as appropriate.

TODO: git ppr seems to be returning non-zero in instances where the
pull requests were successfully created, it seems.