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

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

When Git's source code calls isatty(), it really asks whether the
respective file descriptor is connected to an interactive terminal.

Windows' _isatty() function, however, determines whether the file
descriptor is associated with a character device. And NUL, Windows'
equivalent of /dev/null, is a character device.

Which means that for years, Git mistakenly detected an associated
interactive terminal when being run through the test suite, which
almost always redirects stdin, stdout and stderr to /dev/null.

This bug only became obvious, and painfully so, when the new
bisect--helper entered the `pu` branch and made the automatic build & test
time out because t6030 was waiting for an answer.

For details, see

	https://msdn.microsoft.com/en-us/library/f4s0ddew.aspx

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