revlog: reorder a conditionnal about revlogio

if we are using REVLOGV0, we will not use a rust based index. This small line
movement make it clearer.

Differential Revision: https://phab.mercurial-scm.org/D7830

mercurial/revlog.py

@@ -592,10 +592,10 b' class revlog(object):'
         self._storedeltachains = True
 
         self._io = revlogio()
-        if rustrevlog is not None and self.opener.options.get(b'rust.index'):
-            self._io = rustrevlogio()
         if self.version == REVLOGV0:
             self._io = revlogoldio()
+        elif rustrevlog is not None and self.opener.options.get(b'rust.index'):
+            self._io = rustrevlogio()
         try:
             d = self._io.parseindex(indexdata, self._inline)
         except (ValueError, IndexError):