work around misdetection of stdin attached to a tty

Git on Windows was made aware of the fact that sometimes a file may be
used by another process and so an operation may fail but the user might
be able to fix it and is asking for confirmation whether it should
retry.

This is implemented in a way that git only asks in case stdin and stderr
are attached to a tty. Unfortunately this seems to be misdetected
sometimes causing the testsuite to hang when git is waiting for a user
answer.

This patch works around the situation.

Signed-off-by: Heiko Voigt <hvoigt@hvoigt.net>

t/test-lib.sh

@@ -15,6 +15,10 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see http://www.gnu.org/licenses/ .
 
+# for git on windows so stdin will not be misdetected as attached to a
+# terminal
+exec < /dev/null
+
 # Keep the original TERM for say_color
 ORIGINAL_TERM=$TERM