submodule: use prepare_submodule_repo_env consistently

submodule: use prepare_submodule_repo_env consistently

Before 14111fc (git: submodule honor -c credential.* from
command line, 2016-02-29), it was sufficient for code which
spawned a process in a submodule to just set the child
process's "env" field to "local_repo_env" to clear the
environment of any repo-specific variables.

That commit introduced a more complicated procedure, in
which we clear most variables but allow through sanitized
config. For C code, we used that procedure only for cloning,
but not for any of the programs spawned by submodule.c. As a
result, things like "git fetch --recurse-submodules" behave
differently than "git clone --recursive"; the former will
not pass through the sanitized config.

We can fix this by using prepare_submodule_repo_env()
everywhere in submodule.c.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>