Add option to check whether feature/hotfix/release branch is mergable

[novopl]

What?

Add an optional code path to finish so that it first checks whether the current branch is mergable

Why?

For less git-experienced users it might be easier to just pass that flag every time so they are kept from
accidental merge conflicts.

How?

Add --guard option that toggles the extra code path to test whether the current branch can be merged.

Pull latest base branch, create a temporary branch off the base branch and try to merge the current branch into it. If the merge was successful the branch can be merged