histedit: add verify() to histeditaction

This commits splits the parsing of the histedit rule from its semantic analysis.
It's necessary because sometimes we want to do first without doing the former (reading the
histedit state).

hgext/histedit.py

@@ -346,13 +346,18 b' class histeditaction(object):'
     def fromrule(cls, state, rule):
         """Parses the given rule, returning an instance of the histeditaction.
         """
-        repo = state.repo
         rulehash = rule.strip().split(' ', 1)[0]
+        return cls(state, node.bin(rulehash))
+
+    def verify(self):
+        """ Verifies semantic correctness of the rule"""
+        repo = self.repo
+        ha = node.hex(self.node)
         try:
-            node = repo[rulehash].node()
+            self.node = repo[ha].node()
         except error.RepoError:
-            raise error.Abort(_('unknown changeset %s listed') % rulehash[:12])
-        return cls(state, node)
+            raise error.Abort(_('unknown changeset %s listed')
+                              % ha[:12])
 
     def constraints(self):
         """Return a set of constrains that this action should be verified for
@@ -1175,6 +1180,7 b' def verifyrules(rules, state, ctxs):'
         if verb not in actiontable or verb.startswith('_'):
             raise error.Abort(_('unknown action "%s"') % verb)
         action = actiontable[verb].fromrule(state, rest)
+        action.verify()
         constraints = action.constraints()
         for constraint in constraints:
             if constraint not in _constraints.known():