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>