registry: Hide warning when channel is 'global'.

src/registry.py

@@ -435,6 +435,9 @@ class Value(Group):
         # Also, we're setting them to None instead of raising an error in
         # order not to break existing plugins.
         if channel and not ircutils.isChannel(channel):
+            if channel != 'global':
+                # excluding 'global', it's a special value used for linking
+                # channels and by some plugins.
                 from . import log
                 log.warning(
                     'Trying to get channel-specific value of %s for channel %s, '