optionally find files in another repo

Hi,
Firstly, thanks for writing this package, I enjoy using it every day.

I would like to propose adding the ability to find files in another repo that the current buffer is not part of.

The use-case is that I often have buffers open across many repos. While I'm editing a buffer whose file is part of repo A, I might want to find a file in repo B. But I'd rather not have to switch to a buffer in repo B first in order to invoke helm-ls-git-ls.

Perhaps a new command could be added such as `helm-ls-git-ls-specify-directory` that would be 2 steps: first prompt for the directory (with the current directory as default), and then invoke `helm-ls-git-ls` from within that directory.
Or alternatively, there could be a way to pass a prefix to `helm-ls-git-ls` that would prompt for a directory to search within.