Replace curl-based API calls with helper script

This commit enlarges the role of `git-cppr--github-helper.rb` to include
verifying repository forks, repository branches, and pull request URLs,
allowing this script to replace `curl`-based API calls in `git-pcp.sh`
and `git-ppr.sh`.

This commit also cleans up some messy commented code and unused function
definitions

Finally, this commit adds the `clean.sh` script, which gets rid of the
git-path-friendly copies of scripts that `testenv.sh` creates