chainlint.sed: don't mistake `<< word` in string as here-doc operator

chainlint.sed: don't mistake `<< word` in string as here-doc operator

Tighten here-doc recognition to prevent it from being fooled by text
which looks like a here-doc operator but happens merely to be the
content of a string, such as this real-world case from t7201:

    echo "<<<<<<< ours" &&
    echo ourside &&
    echo "=======" &&
    echo theirside &&
    echo ">>>>>>> theirs"

This problem went unnoticed because chainlint.sed is not a real parser,
but rather applies heuristics to pretend to understand shell code. In
this case, it saw what it thought was a here-doc operator (`<< ours`),
and fell off the end of the test looking for the closing tag "ours"
which it never found, thus swallowed the remainder of the test without
checking it for &&-chain breakage.

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