grep/pcre2: fix an edge case concerning ascii patterns and UTF-8 data

grep/pcre2: fix an edge case concerning ascii patterns and UTF-8 data

If we attempt to grep non-ascii log message text with an ascii pattern, we
run into the following issue:

    $ git log --color --author='.var.*Bjar' -1 origin/master | grep ^Author
    grep: (standard input): binary file matches

So, to fix this teach the grep code to use PCRE2_UTF, as long as the log
output is encoded in UTF-8.

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