Add extra debugging around git.repo gc leaks

We're starting to see this in tests and it's not clear why.

Change-Id: Ic52f74785184b4df0cdc659ee353c44ef7522c89
diff --git a/tests/base.py b/tests/base.py
index 7ee9b9c..4525627 100755
--- a/tests/base.py
+++ b/tests/base.py
@@ -1427,6 +1427,9 @@
         gc.collect()
         for obj in gc.get_objects():
             if isinstance(obj, git.Repo):
+                self.log.debug("Leaked git repo object: %s" % repr(obj))
+                for r in gc.get_referrers(obj):
+                    self.log.debug("  referrer: %s" % repr(r))
                 repos.append(obj)
         self.assertEqual(len(repos), 0)
         self.assertEmptyQueues()