revlog: define the actual index and datafile at loading time

This is just code movement, to make the code closer to where we actually use it
and where it will be defined in the future.

Differential Revision: https://phab.mercurial-scm.org/D10594

mercurial/revlog.py

@@ -315,18 +315,8 b' class revlog(object):'
 
         self.radix = radix
 
-        if postfix is None:
-            indexfile = b'%s.i' % self.radix
-            datafile = b'%s.d' % self.radix
-        elif postfix == b'a':
-            indexfile = b'%s.i.a' % self.radix
-            datafile = b'%s.d' % self.radix
-        else:
-            indexfile = b'%s.i.%s' % (self.radix, postfix)
-            datafile = b'%s.d.%s' % (self.radix, postfix)
-
-        self._indexfile = indexfile
-        self._datafile = datafile
+        self._indexfile = None
+        self._datafile = None
         self._nodemap_file = None
         self.postfix = postfix
         self.opener = opener
@@ -459,6 +449,20 b' class revlog(object):'
     def _loadindex(self):
 
         newversionflags, mmapindexthreshold, force_nodemap = self._init_opts()
+
+        if self.postfix is None:
+            index_file = b'%s.i' % self.radix
+            data_file = b'%s.d' % self.radix
+        elif self.postfix == b'a':
+            index_file = b'%s.i.a' % self.radix
+            data_file = b'%s.d' % self.radix
+        else:
+            index_file = b'%s.i.%s' % (self.radix, self.postfix)
+            data_file = b'%s.d.%s' % (self.radix, self.postfix)
+
+        self._indexfile = index_file
+        self._datafile = data_file
+
         indexdata = b''
         self._initempty = True
         try: