Merge branch 'jk/ref-cache-non-repository-optim'

The underlying machinery used by "ls-files -o" and other commands have been taught not to create empty submodule ref cache for a directory that is not a submodule. This removes a ton of wasted CPU cycles. * jk/ref-cache-non-repository-optim: resolve_gitlink_ref: ignore non-repository paths clean: make is_git_repository a public function