verify: filter messages about missing null manifests (issue2900)

mercurial/verify.py

@@ -176,6 +176,8 b' def _verify(repo):'
 
         for f in sorted(filelinkrevs):
             count += 1
+            if m == nullid:
+                continue
             ui.progress(_('crosschecking'), count, total=total)
             if f not in filenodes:
                 lr = filelinkrevs[f][0]