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

"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