changegroup: don't define lookupmf() until it is needed

lookupmf() is currently defined earlier than when it is needed. Future
patches further refactoring this code will be easier to read when
lookupmf() is in its new home.

mercurial/changegroup.py

@@ -330,6 +330,14 b' class cg1packer(object):'
             mfs.setdefault(c[0], x)
             return x
 
+        for chunk in self.group(clnodes, cl, lookupcl, units=_('changesets'),
+                                reorder=reorder):
+            yield chunk
+        progress(msgbundling, None)
+
+        for f in changedfiles:
+            fnodes[f] = {}
+
         # Callback for the manifest, used to collect linkrevs for filelog
         # revisions.
         # Returns the linkrev node (collected in lookupcl).
@@ -344,13 +352,6 b' class cg1packer(object):'
                         fnodes[f].setdefault(n, clnode)
             return clnode
 
-        for chunk in self.group(clnodes, cl, lookupcl, units=_('changesets'),
-                                reorder=reorder):
-            yield chunk
-        progress(msgbundling, None)
-
-        for f in changedfiles:
-            fnodes[f] = {}
         mfnodes = self.prune(mf, mfs, commonrevs, source)
         for chunk in self.group(mfnodes, mf, lookupmf, units=_('manifests'),
                                 reorder=reorder):