chainlint.pl: recognize test bodies defined via heredoc

chainlint.pl: recognize test bodies defined via heredoc

In order to check tests for semantic problems, chainlint.pl scans test
scripts, looking for tests defined as:

    test_expect_success [prereq] title '
        body
    '

where `body` is a single string which is then treated as a standalone
chunk of code and "linted" to detect semantic issues. (The same happens
for `test_expect_failure` definitions.)

The introduction of test definitions in which the test body is instead
presented via a heredoc rather than as a single string creates a blind
spot in the linting process since such invocations are not recognized by
chainlint.pl.

Prepare for this new style by also recognizing tests defined as:

    test_expect_success [prereq] title - <<\EOT
        body
    EOT

A minor complication is that chainlint.pl has never considered heredoc
bodies significant since it doesn't scan them for semantic problems,
thus it has always simply thrown them away. However, with the new
`test_expect_success` calling sequence, heredoc bodies become
meaningful, thus need to be captured.

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