The final step, split from earlier attempt by Dscho, to loosen the
sideband restriction for now and tighten later at Git v3.0 boundary.
Will retract.
(this branch uses jc/neuter-sideband-fixup.)
* jc/neuter-sideband-post-3.0:
sideband: delay sanitizing by default to Git v3.0
5acbdf83cf