Add support --branch together with --all mentioned in #253

[hever]

[hever]

I'll try to understand you. I'm using subrepo in different use case for me it is meaningful to do this by this way.

Every command I'm using is somehow dangerous. This is life. This one could be dangerous also. But preventing users from doing stupid things is preventing them from doing the smart ones. This command potentially do what you might not want. But it is everything in git, everything is revertible. Need for user interaction is not the way IMHO. There must be good feedback and user must know what has been done.