If we can't write the branch cache, fail quietly.

mercurial/localrepo.py

@@ -322,11 +322,14 b' class localrepository(repo.repository):'
         return self.branchcache
 
     def _writebranchcache(self):
+        try:
         f = self.opener("branches.cache", "w")
         t = self.changelog.tip()
         f.write("%s %s\n" % (hex(t), self.changelog.count() - 1))
         for label, node in self.branchcache.iteritems():
             f.write("%s %s\n" % (hex(node), label))
+        except IOError:
+            pass
 
     def lookup(self, key):
         if key == '.':