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.

from tests import common
import hglib, shutil
from hglib.util import b

class test_init(common.basetest):
    def test_exists(self):
        self.assertRaises(hglib.error.CommandError, hglib.init)

    def test_basic(self):
        self.client.close()
        self.client = None
        shutil.rmtree('.hg')

        self.client = hglib.init().open()
        self.assertTrue(self.client.root().endswith(b('test_init')))