Add missing test file for UTF-16.

The test file that the UTF-16 rejection test looks for is missing, but this went
unnoticed because the test is expected to fail anyway; as a consequence, the
test fails because the file containing the commit message is missing, and not
because the test file contains a NUL byte.  Fix this by including a sample text
file containing a commit message encoded in UTF-16.

Signed-off-by: Brian M. Carlson <sandals@crustytoothpaste.net>
Tested-by: Duy Nguyen <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>