Merge branch 'ps/refstorage-extension' into ps/prompt-parse-HEAD-futureproof

* ps/refstorage-extension:
  t9500: write "extensions.refstorage" into config
  builtin/clone: introduce `--ref-format=` value flag
  builtin/init: introduce `--ref-format=` value flag
  builtin/rev-parse: introduce `--show-ref-format` flag
  t: introduce GIT_TEST_DEFAULT_REF_FORMAT envvar
  setup: introduce GIT_DEFAULT_REF_FORMAT envvar
  setup: introduce "extensions.refStorage" extension
  setup: set repository's formats on init
  setup: start tracking ref storage format
  refs: refactor logic to look up storage backends
  worktree: skip reading HEAD when repairing worktrees
  t: introduce DEFAULT_REPO_FORMAT prereq

diff --cc Documentation/git.txt
Simple merge

diff --cc refs.c
Simple merge

diff --cc refs.h
Simple merge

diff --cc t/test-lib-functions.sh
Simple merge

diff --cc worktree.c
Simple merge