mingw: optionally redirect stderr/stdout via the same handle

mingw: optionally redirect stderr/stdout via the same handle

The "2>&1" notation in Powershell and in Unix shells implies that stderr
is redirected to the same handle into which stdout is already written.

Let's use this special value to allow the same trick with
GIT_REDIRECT_STDERR and GIT_REDIRECT_STDOUT: if the former's value is
`2>&1`, then stderr will simply be written to the same handle as stdout.

The functionality was suggested by Jeff Hostetler.

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