Merge branch 'jc/maint-clean-nested-worktree-in-subdir'

"git clean -d -f" (not "-d -f -f") is supposed to protect nested working
trees of independent git repositories that exist in the current project
working tree from getting removed, but the protection applied only to such
working trees that are at the top-level of the current project by mistake.

* jc/maint-clean-nested-worktree-in-subdir:
  clean: preserve nested git worktree in subdirectories