cmdutil: make walkchangerevs() gracefully handle wdir parents

This code will be completely rewritten, but test-grep.t would fail without
fixing it.

mercurial/cmdutil.py

@@ -2386,12 +2386,20 b' class _followfilter(object):'
 
     def match(self, rev):
         def realparents(rev):
+            try:
             if self.onlyfirst:
                 return self.repo.changelog.parentrevs(rev)[0:1]
             else:
                 return filter(
-                    lambda x: x != nullrev, self.repo.changelog.parentrevs(rev)
+                        lambda x: x != nullrev,
+                        self.repo.changelog.parentrevs(rev),
                 )
+            except error.WdirUnsupported:
+                prevs = [p.rev() for p in self.repo[rev].parents()]
+                if self.onlyfirst:
+                    return prevs[:1]
+                else:
+                    return prevs
 
         if self.startrev == nullrev:
             self.startrev = rev