listkey: display the size of the listkey payload in a debug message

This is a useful information to have in general and we already have debug
output related to listkeys. I'm planning to play around with massive amount of
phases roots and bookmarks so having this data in debug will be very useful.
This already got me to spot that one of the Logilab's review repo is exchanging
65KB of phases data during each exchanges.

mercurial/wireproto.py

@@ -324,6 +324,8 b' class wirepeer(peer.peerrepository):'
         self.ui.debug('preparing listkeys for "%s"\n' % namespace)
         yield {'namespace': encoding.fromlocal(namespace)}, f
         d = f.value
+        self.ui.debug('received listkey for "%s": %i bytes\n'
+                      % (namespace, len(d)))
         yield pushkeymod.decodekeys(d)
 
     def stream_out(self):

tests/test-ssh.t

@@ -457,8 +457,10 b' debug output'
   remote: 1
   preparing listkeys for "bookmarks"
   sending listkeys command
+  received listkey for "bookmarks": 45 bytes
   preparing listkeys for "bookmarks"
   sending listkeys command
+  received listkey for "bookmarks": 45 bytes
   query 1; heads
   sending batch command
   searching for changes
@@ -466,6 +468,7 b' debug output'
   no changes found
   preparing listkeys for "phases"
   sending listkeys command
+  received listkey for "phases": 15 bytes
   checking for updated bookmarks
 
   $ cd ..