t4126: fix "funny directory name" test on Windows (again)

t4126: fix "funny directory name" test on Windows (again)

Even though "git update-index --cacheinfo" ought to be filesystem
agnostic,

    $ git update-index --add --cacheinfo "100644,$empty_blob,funny /empty"

fails only on Windows, and this unfortunately makes the approach of
the previous step unworkable.

Resurrect the earlier approach to give up on running the test on
known-bad platforms.  Instead of computing a custom prerequisite,
just use !MINGW we have used elsewhere.

Signed-off-by: Junio C Hamano <gitster@pobox.com>