Check plugin for updates

Check plugin {RepoName} for available updates by running git fetch in the plugin directory and checking for any unmerged commits. Also reports privacyChanged (the block …/upgrade would land differs materially from the accepted one, or nothing is accepted) and reinstallPrivacyChanged (the same for the block a Reinstall would land: the versions[] branch and pin for this FPP, fetched here as well when it is not the checked-out branch).