linelog: extract `len(self._program)` to a local function

This is a micro optimization prepared for following changes where
`len(self._program)` is used in a loop.

Differential Revision: https://phab.mercurial-scm.org/D4149

mercurial/linelog.py

@@ -308,7 +308,8 b' class linelog(object):'
         else:
             a1info = ar.lines[a1]
             a1inst = self._program[a1info._offset]
-        oldproglen = len(self._program)
+        programlen = self._program.__len__
+        oldproglen = programlen()
         appendinst = self._program.append
 
         # insert