Merge into children update-ref call is too permissive

[orao]

Currently when children are direct ancestors of the new commit that is made we move their branches instead of making a new commit. This behavior should only occur if the branch is an immediate parent and not necessarily any ancestor of the given commit.

The permissive behavior can be useful so we should probably make this a configurable option.

[orao]

There are two situations that need to be considered.

• An empty child stream that is one commit behind and has inherited this change (affected stream with empty diff).
• A source stream that was merged into its parent (promoted from in the previous transaction) which will be a second parent of our parent commit.

These two situations are the only ones I can think of that mean that we can fast-forward the child branch to the parent branch. All other situations should probably be made into cherry-picks. However, I don't know if it is even possible for a different situation to arise at the moment so perhaps issuing a warning would suffice?

Either way this needs to be considered in conjunction with issue #54.

[ghost]

This isn't urgent since it doesn't seem to affect the conversions I've tested with.