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

refs.h

@@ -11,6 +11,9 @@ struct string_list;
 struct string_list_item;
 struct worktree;
 
+unsigned int ref_storage_format_by_name(const char *name);
+const char *ref_storage_format_to_name(unsigned int ref_storage_format);
+
 /*
  * Resolve a reference, recursively following symbolic refererences.
  *