branch create: Don't create or commit if store would reject

[abhinav]

By using the new BranchTx API,
we're able to check whether the store will accept the change
before we actually create the branch.

So, we now first prepare the transaction with the new branch state,
and only if that succeeds do we actually create the branch.

This way, if the store would reject the change
because the base branch is untracked or something similar,
we'll roll back to the original target branch and staged changes.

[abhinav]

This change is part of the following stack:

• branch create: Don't create or commit if store would reject #423 ◀

_{Change managed by git-spice.}