py3: fix comparison between int and None

If stop is None, the condition was always false on Python 2, as None compares
smaller than ints. Therefore we make the condition false if stop is None.

hgext/convert/subversion.py

@@ -817,7 +817,7 b' class svn_source(converter_source):'
                         # We do not know the latest changed revision,
                         # keep the first one with changed paths.
                         break
-                    if revnum <= stop:
+                    if stop is not None and revnum <= stop:
                         break
 
                     for p in paths: