submodule--helper: use submodule_name_to_gitdir in add_submodule

submodule--helper: use submodule_name_to_gitdir in add_submodule

While testing submodule gitdir path encoding, I noticed submodule--helper
is still using a hardcoded modules gitdir path leading to test failures.

Call the submodule_name_to_gitdir() helper instead, which was invented
exactly for this purpose and is already used by all the other locations
which work on gitdirs.

Also narrow the scope of the submod_gitdir_path variable which is not
used anymore in the updated "else" branch.

Signed-off-by: Adrian Ratiu <adrian.ratiu@collabora.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>