Please remember to double-patch anything that should go into 9.1.
For the record, the correct formula seems to be
$ git pull
Current branch master is up to date.
$ git push origin master:refs/heads/REL9_1_STABLE
Total 0 (delta 0), reused 0 (delta 0)
To ssh://git(at)gitmaster(dot)postgresql(dot)org/postgresql.git
* [new branch] master -> REL9_1_STABLE
after which you can check out the branch locally according to whatever
formula you're already using (a separate workdir in my case).
regards, tom lane