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>