notify: make a message translatable

This message has been overlooked by check-code, because it starts with
non-alphabet character ('\').

Making this message translatable seems reasonable, because messages
below in same function are already translatable

  - '\ndiffs (truncated from %d to %d lines):\n\n'
  - '\ndiffs (%d lines):\n\n'

This is also a part of preparation for making "missing _() in ui
message" detection of check-code more exact.

hgext/notify.py

@@ -363,7 +363,7 b' class notifier(object):'
             s = patch.diffstat(difflines)
             # s may be nil, don't include the header if it is
             if s:
-                self.ui.write('\ndiffstat:\n\n%s' % s)
+                self.ui.write(_('\ndiffstat:\n\n%s') % s)
 
         if maxdiff == 0:
             return