mingw: make stderr unbuffered again

mingw: make stderr unbuffered again

When removing the hack for isatty(), we actually removed more than just
an isatty() hack: we removed the hack where internal data structures of
the MSVC runtime are modified in order to redirect stdout/stderr.

Instead of using that hack (that does not work with newer versions of
the runtime, anyway), we replaced it by reopening the respective file
descriptors.

What we forgot was to mark stderr as unbuffered again.

Reported by Hannes Sixt. Fixed with Jeff Hostetler's assistance.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Tested-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>