repopick: update to use latest Gerrit REST API

  * determine the latest patch from the JSON response instead of
    iterating to find it
  * add command line processing via argparse; use -h to see descriptions
  * add --ignore-missing option to not error out on missing project
    paths (e.g. for a device repo used in a large list of gerrits that
    might not be relevant to the particular tester)
  * add --start-branch and --abandon-first options to facilitate a
    workflow where a set of gerrits are cherry-picked into a testing
    branch and then abandoned on the next run of repopick.
  * add --quiet and --verbose options
  * print useful information about the Gerrit including author,
    committer, and subject