t0008: avoid brace expansion

Brace expansion is a shell feature that's not required by POSIX and not
supported by dash nor NetBSD's sh.  Explicitly list all combinations
instead.  Also avoid calling touch by creating the test files with a
redirection instead, as suggested by Junio.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>