chainlint.pl: don't flag broken &&-chain if `$?` handled explicitly

chainlint.pl: don't flag broken &&-chain if `$?` handled explicitly

There are cases in which tests capture and check a command's exit code
explicitly without employing test_expect_code(). They do so by
intentionally breaking the &&-chain since it would be impossible to
capture "$?" in the failing case if the `status=$?` assignment was part
of the &&-chain. Since such constructs are manually checking the exit
code, their &&-chain breakage is legitimate and safe, thus should not be
flagged. Therefore, stop flagging &&-chain breakage in such cases.

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