I considered adding RefTarget template type, but some of the methods naturally
fit to RefName. For example, a conflicted branch name is decorated as "??", so
it makes sense to add branch.conflict() instead of branch.target().conflict().
I'm not pretty sure how many RefName methods we'll need to add to port the
current branch listing, but there will be .tracked(), .tracking_local_present(),
.ahead_by(), and .behind_by().
4db068e7f7