is_submodule_modified(): clear environment properly
is_submodule_modified(): clear environment properly

Rather than only clearing GIT_INDEX_FILE, take the list of environment
variables to clear from local_repo_env, appending the settings for
GIT_DIR and GIT_WORK_TREE.

Signed-off-by: Giuseppe Bilotta <6010d7f2c049984f25cd19568d4ace2cc46b0817@gmail.com>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git