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>