t9800: correct misuse of 'show -s --raw' in a test

t9800: correct misuse of 'show -s --raw' in a test

There is $(git show -s --raw --pretty=format:%at HEAD) in this test
that is meant to grab the author time of the commit.  We used to
have a bug in the command line option parser of the diff family of
commands, where "show -s --raw" was identical to "show -s".

With the "-s" bug fixed, "show -s --raw" would mean the same thing
as "show --raw", i.e. show the output from the diff machinery in the
"raw" format.  And this test will start failing, so fix it before
that happens.

Signed-off-by: Junio C Hamano <gitster@pobox.com>