t/t7811-grep-open.sh: ensure fake "less" is made executable

t/t7811-grep-open.sh: ensure fake "less" is made executable

The fake "less" script was not being made executable.  This can cause the
tests that follow to fail.  This failure is not apparent on platforms which
have DEFAULT_PAGER set to the string "less", since lib-pager.sh will have
set the $less variable to "less" and the SIMPLEPAGER prerequisite will have
been set, and so the "less" script will have already been created properly
and made executable in test 2 'git grep -O'.  On platforms which set
DEFAULT_PAGER to something like "more", no such script will have been
previously created, and tests 7 and 8 will fail.

So, add a call to chmod to make the fake "less" script executable.

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: Junio C Hamano <gitster@pobox.com>