patman: Avoid touching the system or local config

Set the git variables to ensure that any local gitconfig is not used or
affected by the tests.

Signed-off-by: Simon Glass <sjg@chromium.org>

tools/patman/test_common.py

@@ -109,6 +109,9 @@ class TestCommon:
         Returns:
             pygit2.Repository: repository
         """
+        os.environ['GIT_CONFIG_GLOBAL'] = '/dev/null'
+        os.environ['GIT_CONFIG_SYSTEM'] = '/dev/null'
+
         repo = pygit2.init_repository(self.gitdir)
         self.repo = repo
         new_tree = repo.TreeBuilder().write()