Merge branch 'sy/mv-out-of-cone'

Merge branch 'sy/mv-out-of-cone'

"git mv A B" in a sparsely populated working tree can be asked to
move a path from a directory that is "in cone" to another directory
that is "out of cone".  Handling of such a case has been improved.

* sy/mv-out-of-cone:
  builtin/mv.c: fix possible segfault in add_slash()
  mv: check overwrite for in-to-out move
  advice.h: add advise_on_moving_dirty_path()
  mv: cleanup empty WORKING_DIRECTORY
  mv: from in-cone to out-of-cone
  mv: remove BOTH from enum update_mode
  mv: check if <destination> is a SKIP_WORKTREE_DIR
  mv: free the with_slash in check_dir_in_index()
  mv: rename check_dir_in_index() to empty_dir_has_sparse_contents()
  t7002: add tests for moving from in-cone to out-of-cone