Merge branch 'sj/ref-consistency-checks-more'

Merge branch 'sj/ref-consistency-checks-more'

"git fsck" becomes more careful when checking the refs.

* sj/ref-consistency-checks-more:
  builtin/fsck: add `git refs verify` child process
  packed-backend: check whether the "packed-refs" is sorted
  packed-backend: add "packed-refs" entry consistency check
  packed-backend: check whether the refname contains NUL characters
  packed-backend: add "packed-refs" header consistency check
  packed-backend: check if header starts with "# pack-refs with: "
  packed-backend: check whether the "packed-refs" is regular file
  builtin/refs: get worktrees without reading head information
  t0602: use subshell to ensure working directory unchanged