mingw: replace isatty() hack

mingw: replace isatty() hack

Git for Windows has carried a patch that depended on internals
of MSVC runtime, but it does not work correctly with recent MSVC
runtime. A replacement was written originally for compiling
with VC++. The patch in this message is a backport of that
replacement, and it also fixes the previous attempt to make
isatty() tell that /dev/null is *not* an interactive terminal.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Tested-by: Beat Bolli <dev+git@drbeat.li>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>