grep: stess test PCRE v2 on invalid UTF-8 data

grep: stess test PCRE v2 on invalid UTF-8 data

Since my b65abcafc7 ("grep: use PCRE v2 for optimized fixed-string
search", 2019-07-01) we've been dying on invalid UTF-8 data when
grepping for fixed strings if the following are all true:

    * The subject string is non-ASCII (e.g. "ævar")
    * We're under a is_utf8_locale(), e.g. "en_US.UTF-8", not "C"
    * We compiled with PCRE v2
    * That PCRE v2 did not have JIT support

The last of those is why this wasn't caught earlier, per pcre2jit(3):

    "unless PCRE2_NO_UTF_CHECK is set, a UTF subject string is tested
    for validity. In the interests of speed, these checks do not
    happen on the JIT fast path, and if invalid data is passed, the
    result is undefined."

I.e. the subject being matched against our pattern was invalid, but we
were lucky and getting away with it on the JIT path, but the non-JIT
one is stricter.

This patch does nothing to fix that, instead we sneak in support for
fixed patterns starting with "(*NO_JIT)", this disables the PCRE v2
jit with implicit fixed-string matching for testing, see
pcre2syntax(3) the syntax.

This is technically a change in behavior, but it's so obscure that I
figured it was OK. We'd previously consider this an invalid regular
expression as regcomp() would die on it, now we feed it to the PCRE v2
fixed-string path. I thought this was better than introducing yet
another GIT_TEST_* environment variable.

We're also relying on a behavior of PCRE v2 that technically could
change, but I think the test coverage is worth dipping our toe into
some somewhat undefined behavior.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>