more git commands needed

[kretes]

we would benefit from more git commands in Git DSL:

1. create branch from revision/tag
2. push specific branch

we currently have some rough implementation of them in class extending Git

def branchFromTag(name: String, tag: String) = cmd("branch", name, tag)

def pushBranch(branchName: String) = cmd("push", "origin", branchName)