We need to extend mgarepo to handle the backports branch as it does with the updates branch.

There are at least two things that mgarepo should be able to do:
- Checkout a backports branch using a simple syntax, i.e. mgarepo co backports/4/pkgname
- If the branch does not exist, create the actual backports branch for the given package, following this scheme:
  o Branch off the updates/4/pkgname branch to backports/4/pkgname
  o If pkgname is new, branch off cauldron/pkgname to backports/4/pkgname

For the first point, I've done a patch (arguably not the best implementation though) that mimics the behaviour of mgarepo co 4/pkgname and extends it to support backports.

I've tested it locally and it works fine.

Reproducible: 

Steps to Reproduce: