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: Nguyễn Thái Ngọc Duy <pclouds@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>