changegroup: clear progress callback after changelog processing

The progress callback is replaced by one for manifests after changelog
processing is done, but let's not depend on manifests replacing the
value and instead explicitly clear it.

mercurial/changegroup.py

@@ -376,6 +376,7 b' class cg1unpacker(object):'
                 clend = len(cl)
                 changesets = clend - clstart
                 repo.ui.progress(_('changesets'), None)
+                self.callback = None
 
                 # pull off the manifest group
                 repo.ui.status(_("adding manifests\n"))