[meld] Clear old chunk cache when clearing merge cache (closes bgo#693690)

[Kai Willadsen <kaiw src gnome org>]

commit b4df00d3e32863368b5e9dd1d90d1d7fbc964659
Author: Kai Willadsen <kai willadsen gmail com>
Date:   Thu Apr 18 06:20:29 2013 +1000

    Clear old chunk cache when clearing merge cache (closes bgo#693690)

 meld/diffutil.py | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/meld/diffutil.py b/meld/diffutil.py
index b5750d4..ea921ea 100644
--- a/meld/diffutil.py
+++ b/meld/diffutil.py
@@ -473,4 +473,5 @@ class Differ(gobject.GObject):
         self.diffs = [[], []]
         self.seqlength = [0] * self.num_sequences
         self._initialised = False
+        self._old_merge_cache = set()
         self._update_merge_cache([""] * self.num_sequences)