git-p4: use marshal format version 2 when sending to p4

p4 does not appear to understand marshal format version 3 and above.
Version 2 was the latest supported by python-2.7.

Signed-off-by: Yang Zhao <yang.zhao@skyboxlabs.com>
Reviewed-by: Ben Keene <seraphire@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-p4.py

@@ -1679,7 +1679,8 @@ class P4Submit(Command, P4UserMap):
         c = changes[0]
         if c['User'] == newUser: return   # nothing to do
         c['User'] = newUser
-        input = marshal.dumps(c)
+        # p4 does not understand format version 3 and above
+        input = marshal.dumps(c, 2)
 
         result = p4CmdList("change -f -i", stdin=input)
         for r in result: