exchange: allow passing no includes/excludes to `pull()`

I would expect that `exchange.pull(includepats=[])` results in an empty list of
include patterns to be passed to the remote, but it doesn't currently because we
check for any truthy value instead of checking specifically for `not None`.

mercurial/exchange.py

@@ -1637,7 +1637,7 b' def pull('
 
     # We allow the narrow patterns to be passed in explicitly to provide more
     # flexibility for API consumers.
-    if includepats or excludepats:
+    if includepats is not None or excludepats is not None:
         includepats = includepats or set()
         excludepats = excludepats or set()
     else: