mingw: add experimental feature to redirect standard handles

mingw: add experimental feature to redirect standard handles

Particularly when calling Git from applications, such as Visual Studio's
Team Explorer, it is important that stdin/stdout/stderr are closed
properly. However, when spawning processes on Windows, those handles
must be marked as inheritable if we want to use them, but that flag is a
global flag and may very well be used by other spawned processes which
then do not know to close those handles.

Let's introduce a set of environment variables (GIT_REDIRECT_STDIN and
friends) that specify paths to files, or even better, named pipes (which
are similar to Unix sockets) and that are used by the spawned Git
process.  This helps work around above-mentioned issue: those named
pipes will be opened in a non-inheritable way upon startup, and no
handles are passed around (and therefore no inherited handles need to be
closed by any spawned child).

This feature shipped with Git for Windows (marked as experimental) since
v2.11.0(2), so it has seen some serious testing in the meantime.

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