Fix order of operators before executing the git command

Since Python 3.3, the hash value of an object is seeded randomly, making it
change between each call. As a consequence, the `dict` type relying on the hash
value for the order of the items upon iterating on it, and the parameters
passed to `git` being passed as `kwargs` to the `execute()` method, the order
of parameters will change randomly between calls.

For example, when you call `git.remote.pull()` in a code, two consecutives run
will generate:

1. git pull --progress -v origin master
2. git pull -v --progress origin master

Within the `transform_kwargs()` method, I'm promoting `kwargs` into an
`collections.OrderedDict` being built with `kwargs` sorted on the keys.
Then it will ensure that each subsequent calls will execute the
parameters in the same order.

git/cmd.py

@@ -13,6 +13,8 @@
 import errno
 import mmap
 
+from collections import OrderedDict
+
 from contextlib import contextmanager
 import signal
 from subprocess import (
@@ -783,6 +785,7 @@ def transform_kwarg(self, name, value, split_single_char_options):
     def transform_kwargs(self, split_single_char_options=True, **kwargs):
         """Transforms Python style kwargs into git command line options."""
         args = list()
+        kwargs = OrderedDict(sorted(kwargs.items(), key=lambda x: x[0]))
         for k, v in kwargs.items():
             if isinstance(v, (list, tuple)):
                 for value in v: