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

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>