t7800: avoid arithmetic expansion notation

t7800: avoid arithmetic expansion notation

The construct "var=$(( something ..." is interpreted by some shells as
arithmetic expansion, even when it clearly is not, e.g.

	var=$((foo; bar) | baz)

Avoid the issue by giving an extra SP to help the parser, i.e.

	var=$( (foo; bar) | baz )

Noticed by Michael J Gruber.

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