revlog: raise WdirUnsupported when wdirrev is passed

revlog.parentrevs() is called while evaluating ^ operator in revsets. When wdir
is passed, it raises IndexError. This patch raises WdirUnsupported if wdir is
passed in the function. The error will be caugth in future patches.

mercurial/revlog.py

@@ -26,6 +26,7 b' from .node import ('
     hex,
     nullid,
     nullrev,
+    wdirrev,
 )
 from .i18n import _
 from . import (
@@ -481,7 +482,12 b' class revlog(object):'
         return self.index[rev][4]
 
     def parentrevs(self, rev):
-        return self.index[rev][5:7]
+        try:
+            return self.index[rev][5:7]
+        except IndexError:
+            if rev == wdirrev:
+                raise error.WdirUnsupported
+            raise
 
     def node(self, rev):
         return self.index[rev][7]