Allow --no-thin option for git push

Add workaround for long-lasting bug with git optimization "thin push" and gerrit.
* Option "disablethinpush" added to config file parsing (default: false)
* Option "--no-thin" added to command-line parsing (ex: git review --no-thin)

Referenced: https://bugs.launchpad.net/git-review/+bug/1332549
Issue: 1332549