'pulls checkout' is safe and re-entrant

* Can be safely called from any current branch
* Can be safely called several times in a row
* Re-pushes are always taken into account
* No risk of merging into an old local master
* Doesn't impose a branch naming scheme: you get an anonymous branch and
decide what to do with it.
Merge pull request #100 from shykes/pulls-checkout-reentrant

'pulls checkout' is safe and re-entrant