git_dir must always be non-NULL so "if (git_dir)" is unnecessary.
Before this code, if git_dir == NULL, it will default to
DEFAULT_GIT_DIR_ENVIRONMENT.
Signed-off-by: NNguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>