set unassigned stdin to improve pyinstaller compatibility

To create a window application with pyinstaller, all suprocess input and output streams must be assigned and must not be None.
https://stackoverflow.com/a/51706087/7076612

Author: smokephil

git/cmd.py

@@ -12,7 +12,8 @@
 from subprocess import (
     call,
     Popen,
-    PIPE
+    PIPE,
+    DEVNULL
 )
 import subprocess
 import threading
@@ -873,7 +874,7 @@ def execute(self,
                          env=env,
                          cwd=cwd,
                          bufsize=-1,
-                         stdin=istream,
+                         stdin=istream or DEVNULL,
                          stderr=PIPE,
                          stdout=stdout_sink,
                          shell=shell is not None and shell or self.USE_SHELL,