Code clean-up.

* ma/worktree-cleanups:
  worktree: use skip_prefix to parse target
  worktree: rename copy-pasted variable
  worktree: update renamed variable in comment
  worktree: inline `worktree_ref()` into its only caller
  wt-status: introduce wt_status_state_free_buffers()
  wt-status: print to s->fp, not stdout
  wt-status: replace sha1 mentions with oid