Migration guide: Track correct develop branch

[tobiasdiez]

If you have an existing clone of the sage repo that is still connected to the trac git repo, one needs to change it to track the new github repo. I think this should be something like

$ git fetch upstream
$ git branch -u upstream/develop develop
$ git remote set-head upstream -a

(or replace upstream by origin if you want to track the develop branch of your fork for some reason)

[kwankyu]

I used git branch -D develop; git branch --track develop upstream/develop, but yours look simpler.