t4210: skip more command-line encoding tests on MinGW

t4210: skip more command-line encoding tests on MinGW

In 5212f91deb ("t4210: skip command-line encoding tests on mingw",
2014-07-17) the positive tests in this file were skipped. That left
the negative tests that don't produce a match.

An upcoming change to migrate the "fixed" backend of grep to PCRE v2
will cause these "log" commands to produce an error instead on
MinGW. This is because the command-line on that platform implicitly
has its encoding changed before being passed to git. See [1].

1. https://public-inbox.org/git/nycvar.QRO.7.76.6.1907011515150.44@tvgsbejvaqbjf.bet/

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