Merge branch 'ps/use-reftable-as-default-in-3.0'

The reftable ref backend has matured enough; Git 3.0 will make it the default format in a newly created repositories by default. * ps/use-reftable-as-default-in-3.0: setup: use "reftable" format when experimental features are enabled BreakingChanges: announce switch to "reftable" format