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

diff --cc repository.h
Simple merge

diff --cc setup.c
Simple merge