t9902-completion.sh: old Bash still does not support array+=('') notation

t9902-completion.sh: old Bash still does not support array+=('') notation

Old Bash (3.0) which is distributed with RHEL 4.X and other ancient
platforms that are still in wide use, does not understand the
array+=() notation.  Let's use an explicit assignment to the new array
element which works everywhere, like:

   array[${#array[@]}+1]=''

The right-hand side '' is not strictly necessary, but in this case
I think it is more clear.

Signed-off-by: Brandon Casey <drafnel@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>