set default file encoding to utf-8

There's no reason to support any other encoding in these files.
This only affects the files themselves and not streams they open.

Bug: https://crbug.com/gerrit/10418
Change-Id: I053cb40cd3666ce5c8a0689b9dd938f24ca765bf

tests/test_git_config.py

@@ -1,3 +1,5 @@
+# -*- coding:utf-8 -*-
+
 import os
 import unittest
 

tests/test_wrapper.py

@@ -1,3 +1,4 @@
+# -*- coding:utf-8 -*-
 #
 # Copyright (C) 2015 The Android Open Source Project
 #