"git worktree" learned move and remove subcommands.

* nd/worktree-move:
  t2028: fix minor error and issues in newly-added "worktree move" tests
  worktree remove: allow it when $GIT_WORK_TREE is already gone
  worktree remove: new command
  worktree move: refuse to move worktrees with submodules
  worktree move: accept destination as directory
  worktree move: new command
  worktree.c: add update_worktree_location()
  worktree.c: add validate_worktree()