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>

t/t7800-difftool.sh

@@ -289,7 +289,7 @@ test_expect_success PERL 'setup with 2 files different' '
 '
 
 test_expect_success PERL 'say no to the first file' '
-	diff=$((echo n; echo) | git difftool -x cat branch) &&
+	diff=$( (echo n; echo) | git difftool -x cat branch ) &&
 
 	echo "$diff" | stdin_contains m2 &&
 	echo "$diff" | stdin_contains br2 &&
@@ -298,7 +298,7 @@ test_expect_success PERL 'say no to the first file' '
 '
 
 test_expect_success PERL 'say no to the second file' '
-	diff=$((echo; echo n) | git difftool -x cat branch) &&
+	diff=$( (echo; echo n) | git difftool -x cat branch ) &&
 
 	echo "$diff" | stdin_contains master &&
 	echo "$diff" | stdin_contains branch &&