[firefox-git] switch from moz-git-tools to git-cinnabar

[jankeromnes]

This is the preferred way of using Git for Firefox nowadays.

However, it doesn't seem to work quite well with the https://github.com/mozilla/gecko-dev repository (incompatible SHAs?), so we need to make sure our cinnabar setup makes every Firefox/Git developer happy.

[jankeromnes]

@nt1m mentioned we now have https://github.com/mozilla/gecko as well, which solves git-cinnabar's GitHub-incompatibility issue.