chainlint: recognize multi-line $(...) when command cuddled with "$("

For multi-line $(...) expressions nested within subshells, chainlint.sed
only recognizes:

    x=$(
        echo foo &&
        ...

but it is not unlikely that test authors may also cuddle the command
with the opening "$(", so support that style, as well:

    x=$(echo foo &&
        ...

The closing ")" is already correctly recognized when cuddled or not.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/chainlint/multi-line-nested-command-substitution.test

@@ -6,4 +6,13 @@
 	) &&
 	echo ok
 ) |
-sort
+sort &&
+(
+	bar &&
+	x=$(echo bar |
+		cat
+	) &&
+	y=$(echo baz |
+		fip) &&
+	echo fail
+)