revlog: move loading of index data into own method

This will allow us to "reload" a revlog instance from a rewritten
index file, which will be used in a subsequent commit.

Differential Revision: https://phab.mercurial-scm.org/D4868

mercurial/revlog.py

@@ -415,6 +415,9 b' class revlog(object):'
             raise error.RevlogError(_('revlog chunk cache size %r is not a '
                                       'power of 2') % self._chunkcachesize)
 
+        self._loadindex(v, mmapindexthreshold)
+
+    def _loadindex(self, v, mmapindexthreshold):
         indexdata = ''
         self._initempty = True
         try: