Extend `enum MergeTarget` to include a new variant, `Branch`.
The intended use of this new variant is to allow a `Patch` to have a
target branch other than the default branch.
The `Branch` variant holds a `TargetBranch` which, in turn, is ensured
to be a `Qualified` reference that begins with `refs/heads`, i.e. a
Git branch.
f0c6abc6ea