Add hub check to git-pcp, cherry-pick from PR

This commit adds more hub-dependent features to git-pcp and splits out
some common requirements into a support file. It also adds support for
$GITHUB_HOST

* git-require-hub.sh encapsulates the GITHUB_URL resolution and hub
  executable requirements

* git-pcp.sh now supports cherry-picking from a pull request - github
  PR URLs can be included in the list of commits alongside standard
  git-style commit refs

* git-ppr.sh has been updated to use the git-require-hub