client: raise KeyError from __getitem__ (BC) object.__getitem__ is supposed to raise either IndexError or KeyError (depending on whether the thing is a sequence or a mapping). Before, we would raise ValueError because that's what the context constructor raises. I choose to raise KeyError because IndexError felt a bit too limiting. This does sacrifice some magic with for loops handling IndexError. However, iteration of this object should be handled by a custom __iter__, so I don't think this is a problem.

--- a/tests/test-context.py	Wed Jul 06 14:11:02 2016 -0700
+++ b/tests/test-context.py	Wed Jul 06 14:14:22 2016 -0700
@@ -17,6 +17,8 @@
         self.append('c', 'c')
         rev1, node1 = self.client.commit(b('second'), addremove=True)
 
+        self.assertRaises(KeyError, self.client.__getitem__, 'doesnotexist')
+
         ctx = self.client[node0]
 
         self.assertEquals(ctx.description(), b('first'))