--bump does not honor branch settings

[Raskil]

Hi,

msync does work as expected in normal cases (msync update).
As soon as I add --bump to msync update, it fails because the bump method tries to update the master branch, no matter what branches you specify.
My modules do not have a master branch at all.

Regards

Raskil

[neomilium]

@Raskil As you are a user of bump feature, could you contribute to the discussion started at #216