fix(blame): Support Python 3.7 in the test suite

Apparently the public attributes of a `MagicMock` changed between 3.7 and 3.8,
which was causing the unit test for `rev_opts` to fail.

The unit test has been improved to use more stable APIs so that it can pass in
all versions of Python supported by GitPython.

Author: thehale

test/test_repo.py

@@ -557,8 +557,8 @@ def test_blame_complex_revision(self, git):
     def test_blame_accepts_rev_opts(self, git):
         res = self.rorepo.blame("HEAD", "README.md", rev_opts=["-M", "-C", "-C"])
         expected_args = ['blame', 'HEAD', '-M', '-C', '-C', '--', 'README.md']
-        for idx, arg in enumerate(expected_args):
-            self.assertEqual(git.call_args.args[idx], arg)
+        boilerplate_kwargs = {"p" : True, "stdout_as_string": False}
+        git.assert_called_once_with(*expected_args, **boilerplate_kwargs)
 
     @skipIf(
         HIDE_WINDOWS_KNOWN_ERRORS and Git.is_cygwin(),