Merge branch 'ps/refstorage-extension'

Introduce a new extension "refstorage" so that we can mark a
repository that uses a non-default ref backend, like reftable.

* 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 builtin/clone.c
Simple merge

diff --cc builtin/init-db.c
index b89814a6f87da30fc1721ecef02637f61600cd32,a4f81e2af591ab75053ec9693b980a49c8280356..0170469b849e5e5ad52cf8586f4540088b0edd61
--- 1/builtin/init-db.c
--- 2/builtin/init-db.c
+++ b/builtin/init-db.c
@@@ -10,6 -11,7 +10,8 @@@
  #include "object-file.h"
  #include "parse-options.h"
  #include "path.h"
+ #include "refs.h"
++#include "repository.h"
  #include "setup.h"
  #include "strbuf.h"
  

diff --cc refs.c
Simple merge

diff --cc refs.h
Simple merge

diff --cc refs/files-backend.c
Simple merge

diff --cc refs/packed-backend.c
Simple merge

diff --cc setup.c
Simple merge

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

diff --cc worktree.c
Simple merge