pickaxe -S: support content with NULs under --pickaxe-regex

pickaxe -S: support content with NULs under --pickaxe-regex

Fix a bug in the matching routine powering -S<rx> --pickaxe-regex so
that we won't abort early on content that has NULs in it.

We've had a hard requirement on REG_STARTEND since 2f8952250a8 (regex:
add regexec_buf() that can work on a non NUL-terminated string,
2016-09-21), but this sanity check dates back to d01d8c67828 (Support
for pickaxe matching regular expressions, 2006-03-29).

It wasn't needed anymore, and as the now-passing test shows, actively
getting in our way. Since we always require REG_STARTEND support we do
not need to stop at NULs. If we are dealing with a haystack with NUL
in it. The needle may be behind that NUL.

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