environment.c: fix potential segfault by get_git_common_dir()

setup_git_env() must be called before this function to initialize git_common_dir so that it returns a non NULL string. And it must return a non NULL string or segfault can happen because all callers expect so. It does not do so explicitly though and depends on get_git_dir() being called first (which will guarantee setup_git_env()). Avoid this dependency and call setup_git_env() by itself. test-ref-store.c will hit this problem because it's very lightweight, just enough initialization to exercise refs code, and get_git_dir() will never be called until get_worktrees() is, which uses get_git_common_dir and hits a segfault. Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>