Merge branch 'jh/notes-merge-in-git-dir-worktree'

Running "notes merge --commit" failed to perform correctly when run
from any directory inside $GIT_DIR/.  When "notes merge" stops with
conflicts, $GIT_DIR/NOTES_MERGE_WORKTREE is the place a user edits
to resolve it.

By Johan Herland (3) and Junio C Hamano (1)
* jh/notes-merge-in-git-dir-worktree:
  notes-merge: Don't remove .git/NOTES_MERGE_WORKTREE; it may be the user's cwd
  notes-merge: use opendir/readdir instead of using read_directory()
  t3310: illustrate failure to "notes merge --commit" inside $GIT_DIR/
  remove_dir_recursively(): Add flag for skipping removal of toplevel dir