update submodules: move up prepare_submodule_repo_env

update submodules: move up prepare_submodule_repo_env

In a later patch we need to prepare the submodule environment with
another git directory, so split up the function.

Also move it up in the file such that we do not need to declare the
function later before using it.

Signed-off-by: Stefan Beller <redacted>
Signed-off-by: Junio C Hamano <redacted>