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>