test-lib: allow test snippets as here-docs

Most test snippets are wrapped in single quotes, like:

  test_expect_success 'some description' '
          do_something
  '

This sometimes makes the snippets awkward to write, because you can't
easily use single quotes within them. We sometimes work around this with
$SQ, or by loosening regexes to use "." instead of a literal quote, or
by using double quotes when we'd prefer to use single-quotes (and just
adding extra backslash-escapes to avoid interpolation).

This commit adds another option: feeding the snippet via the function's
stdin. This doesn't conflict with anything the snippet would want to do,
because we always redirect its stdin from /dev/null anyway (which we'll
continue to do).

A few notes on the implementation:

  - it would be nice to push this down into test_run_, but we can't, as
    test_expect_success and test_expect_failure want to see the actual
    script content to report it for verbose-mode. A helper function
    limits the amount of duplication in those callers here.

  - The helper function is a little awkward to call, as you feed it the
    name of the variable you want to set. The more natural thing in
    shell would be command substitution like:

      body=$(body_or_stdin "$2")

    but that loses trailing whitespace. There are tricks around this,
    like:

      body=$(body_or_stdin "$2"; printf .)
      body=${body%.}

    but we'd prefer to keep such tricks in the helper, not in each
    caller.

  - I implemented the helper using a sequence of "read" calls. Together
    with "-r" and unsetting the IFS, this preserves incoming whitespace.
    An alternative is to use "cat" (which then requires the gross "."
    trick above). But this saves us a process, which is probably a good
    thing. The "read" builtin does use more read() syscalls than
    necessary (one per byte), but that is almost certainly a win over a
    separate process.

    Both are probably slower than passing a single-quoted string, but
    the difference is lost in the noise for a script that I converted as
    an experiment.

  - I handle test_expect_success and test_expect_failure here. If we
    like this style, we could easily extend it to other spots (e.g.,
    lazy_prereq bodies) on top of this patch.

  - even though we are using "local", we have to be careful about our
    variable names. Within test_expect_success, any variable we declare
    with local will be seen as local by the test snippets themselves (so
    it wouldn't persist between tests like normal variables would).

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

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