add git command options

Add __call__ method to Git object to allow
passing git command options to the executable
requires flag to transform_kwargs
add unit tests

Change-Id: If1bc01008e66d3fd3811c15b56e58f38c95b9887

Author: Eric Brunson

git/test/test_git.py

@@ -119,3 +119,5 @@ def test_single_char_git_options_are_passed_to_git(self):
         output_value = self.git(c='user.name={}'.format(input_value)).config('--get', 'user.name')
         self.assertEquals(input_value, output_value)
 
+    def test_change_to_transform_kwargs_does_not_break_command_options(self):
+        self.git.log(n=1)