grr could check that 'master' is up to date

[jlevon]

If I already have a grr setup for a branch I'm working on, then I do 'git fetch' then pull --rebase into that branch, but I don't update my local master branch, then I end up with a Gerrit CR with all the intervenening changes I just pulled in. It'd be nice if grr could check for this and error out?