Clean up some useless comments, output enhancements

* all warning messages now use `warn` so they go to `stderr`

* all (?) info messages now use `say` so they can be silenced with
  `$GIT_QUIET`
Add --skip to cppr, retries to ppr verify_forks

`git cppr --skip` now calls the appropriate subcommand with `--skip`

Function `verify_forks` in `git-ppr.sh` now retries the github api
request up to a threshold