Prevent type exception on concatenation if diffstat returns None.

This will most often occur if diffstat is not installed in the
target platform, though may also happen in other cases where
diffstat fails to execute.

Signed-off-by: Sean Dague <[e-mail]>

hgext/notify.py

@@ -241,7 +241,9 b' class notifier(object):'
         difflines = self.ui.popbuffer().splitlines(1)
         if self.ui.configbool('notify', 'diffstat', True):
             s = patch.diffstat(difflines)
-            self.ui.write('\ndiffstat:\n\n' + s)
+            # s may be nil, don't include the header if it is
+            if s:
+                self.ui.write('\ndiffstat:\n\n%s' % s)
         if maxdiff > 0 and len(difflines) > maxdiff:
             self.ui.write(_('\ndiffs (truncated from %d to %d lines):\n\n') %
                           (len(difflines), maxdiff))