Update the isatty() emulation for Windows by updating the previous
hack that depended on internals of (older) MSVC runtime.
* js/mingw-isatty:
mingw: replace isatty() hack
mingw: fix colourization on Cygwin pseudo terminals
mingw: adjust is_console() to work with stdin
mingw: intercept isatty() to handle /dev/null as Git expects it