git-submodule.sh: separate parens by a space to avoid confusing some shells

git-submodule.sh: separate parens by a space to avoid confusing some shells

Some shells interpret '(( ))' according to the rules for arithmetic
expansion.  This may not follow POSIX, but is prevalent in commonly used
shells.  Bash does not have a problem with this particular instance of
'((', likely because it is not followed by a '))', but the public domain
ksh does, and so does ksh on IRIX 6.5.

So, add a space between the parenthesis to avoid confusing these shells.

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