pre-commit.sample: Diff against the empty tree when HEAD is invalid

This was already the case for the old "diff --check" call, but the new
one that checks whether there are any non-ascii file names was missing
it, making that check fail for root commits.

Signed-off-by: Björn Steinbrink <B.Steinbrink@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>