Add chstream-action option

[orao]

There is a number of things that we could map the chstream transaction to, none of which are ideal so allowing the end user to choose what happens might be best.

We could do a git rebase with some merge strategy, a git cherry-pick or git merge depending on what has happened and on what new commit we are basing our changes on.

The most accurate representation is the git merge but it results in an ugly Git graph which means that it would be good to make it optional.

Additionally, we could try a fast-forward first followed by an attempt to rebase, followed by a merge or cherry-pick... Which should make it look pretty in git but may not be the most representative so we probably need to add some flags here.