Merge branch 'js/mingw-isatty' into maint

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