The code to traverse objects for reachability, used to decide what
objects are unreferenced and expendable, have been taught to also
consider per-worktree refs of other worktrees as starting points to
prevent data loss.

* nd/per-worktree-ref-iteration:
  git-worktree.txt: correct linkgit command name
  reflog expire: cover reflog from all worktrees
  fsck: check HEAD and reflog from other worktrees
  fsck: move fsck_head_link() to get_default_heads() to avoid some globals
  revision.c: better error reporting on ref from different worktrees
  revision.c: correct a parameter name
  refs: new ref types to make per-worktree refs visible to all worktrees
  Add a place for (not) sharing stuff between worktrees
  refs.c: indent with tabs, not spaces