support scripts for gerrit in MG

These are not included in the public mudlib, but may be
provided on request.

gerrit-create-project: called from gerrit, creates sync repo
               and imports stuff from mud
gerrit-import: called by cron, creates new projects for dirs
               with gerrit-create-project files
gerrit-autocommit: called by cron, calls gerrit-sync2lib for
               every gerrit repo
gerrit-sync2lib: called by gerrit and gerrit-autocommit, syncs
               between gerrit repos and mudlib
gerrit.env: set environment variables for gerrit scripts