revlog: allow to pass an existing docket to `_loadindex()`

This will be when switching undelying files, for examples during censors and strip operation with rvlog-v2.

Differential Revision: https://phab.mercurial-scm.org/D10800

mercurial/revlog.py

@@ -487,7 +487,7 b' class revlog(object):'
                 raise
             return b''
 
-    def _loadindex(self):
+    def _loadindex(self, docket=None):
 
         new_header, mmapindexthreshold, force_nodemap = self._init_opts()
 
@@ -498,6 +498,10 b' class revlog(object):'
         else:
             entry_point = b'%s.i' % self.radix
 
+        if docket is not None:
+            self._docket = docket
+            self._docket_file = entry_point
+        else:
         entry_data = b''
         self._initempty = True
         entry_data = self._get_data(entry_point, mmapindexthreshold)
@@ -537,6 +541,8 b' class revlog(object):'
                 self._docket = docketutil.parse_docket(
                     self, entry_data, use_pending=self._trypending
                 )
+
+        if self._docket is not None:
             self._indexfile = self._docket.index_filepath()
             index_data = b''
             index_size = self._docket.index_end