dulwich.client: fix fetch if no progress function is specified

Change-Id: I0627b9bea8d7259984fb6587a877bb027dde7367

dulwich/client.py

@@ -240,7 +240,8 @@ class GitClient(object):
             if channel == 1:
                 pack_data(pkt)
             elif channel == 2:
-                progress(pkt)
+                if progress is not None:
+                    progress(pkt)
             else:
                 raise AssertionError("Invalid sideband channel %d" % channel)
         return refs