Merge branch 'js/mingw-isatty'

We often decide if a session is interactive by checking if the
standard I/O streams are connected to a TTY, but isatty() emulation
on Windows incorrectly returned true if it is used on NUL (i.e. an
equivalent to /dev/null). This has been fixed.

* js/mingw-isatty:
  mingw: intercept isatty() to handle /dev/null as Git expects it