Merge branch 'nd/worktree-prune'

The way "git worktree prune" worked internally has been simplified,
by assuming how "git worktree move" moves an existing worktree to a
different place.

* nd/worktree-prune:
  worktree prune: improve prune logic when worktree is moved
  worktree: delete dead code
  gc.txt: more details about what gc does
