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>

environment.c

@@ -214,6 +214,8 @@ const char *get_git_dir(void)
 
 const char *get_git_common_dir(void)
 {
+	if (!git_dir)
+		setup_git_env();
 	return git_common_dir;
 }