verify: add a note about a paleo-bug

In the very early days of hg, it was possible to commit /dev/null because our
patch importer was too simple. Repos from this era may still
exist, add a note about why we ignore this name.

mercurial/verify.py

@@ -169,7 +169,7 b' def _verify(repo):'
             for f, fn in mf.readdelta(n).iteritems():
                 if not f:
                     err(lr, _("file without name in manifest"))
-                elif f != "/dev/null":
+                elif f != "/dev/null": # ignore this in very old repos
                     filenodes.setdefault(_normpath(f), {}).setdefault(fn, lr)
         except Exception, inst:
             exc(lr, _("reading manifest delta %s") % short(n), inst)