clone: concurrent interactive authentication

When doing a multi-job recurse-submodules clone on a repo with http submodules that require interactive authentication, all submodule clones happen in parallel and the username password prompts are all printed at once, including disabling echo, making it effectively unusable as you don't know if you are entering a username or password. I think this could be fixed by passing some shared memory with a mutex to each of the parallel job processes, which they can then lock on if they do prompt for credentials. Since there is the workaround of omitting the -j, is this change worth making and would it be accepted?