hg fix -r abc123 will format that commit but not its
descendants. That seems expected given the option name (-r), but
it's very rarely what the user wants to do. The problem is that any
descendants of that commit will not be formatted, leaving them as
orphans that are hard to evolve. They are hard to evolve because the
new parent will have formatting changes that the orphan doesn't have.
I talked to Danny Hooper (who wrote most of the fix extension) about
the problem and we agreed that deprecating -r in favor of a new -s
argument (mimicing rebase's -s) would be a good way of reducing the
risk that users end up with these hard-to-evolve orphans. So that's
what this patch implements.