submodule: export sanitized GIT_CONFIG_PARAMETERS

submodule: export sanitized GIT_CONFIG_PARAMETERS

Commit 14111fc (git: submodule honor -c credential.* from
command line, 2016-02-29) taught git-submodule.sh to save
the sanitized value of $GIT_CONFIG_PARAMETERS when clearing
the environment for a submodule. However, it failed to
export the result, meaning that it had no effect for any
sub-programs.

We didn't catch this in our initial tests because we checked
only the "clone" case, which does not go through the shell
script at all. Provoking "git submodule update" to do a
fetch demonstrates the bug.

Noticed-by: Lars Schneider <larsxschneider@gmail.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>