verify: recover lost freeing of memory

In df8973e1fb45 (verify: move file cross checking to its own function,
2016-01-05), "mflinkrevs = None" was moved into function, so the
reference was cleared there, but the calling function now held on to
the variable. The point of clearing it was presumably to free up
memory, so let's move the clearing to the calling function where it
makes a difference. Also change "mflinkrevs = None" to "del
mflinkrevs", since the comment about scope now surely is obsolete.

mercurial/verify.py

@@ -149,6 +149,7 b' class verifier(object):'
         filenodes = self._verifymanifest(mflinkrevs)
 
         self._crosscheckfiles(mflinkrevs, filelinkrevs, filenodes)
+        del mflinkrevs
 
         totalfiles, filerevisions = self._verifyfiles(filenodes, filelinkrevs)
 
@@ -249,7 +250,6 b' class verifier(object):'
                 ui.progress(_('crosschecking'), count, total=total)
                 self.err(c, _("changeset refers to unknown manifest %s") %
                          short(m))
-            mflinkrevs = None # del is bad here due to scope issues
 
             for f in sorted(filelinkrevs):
                 count += 1