namespaces: add __getitem__ property
Since the namespaces object uses an underlying (sorted) dictionary to store the
namespaces, it makes sense to expose this to naturally gain access to those
namespaces.
$ hg init
$ touch unknown
$ touch a
$ hg add a
$ hg ci -m "1"
$ touch b
$ hg add b
$ hg ci -m "2"
Should show unknown
$ hg status
? unknown
$ hg revert -r 0 --all
removing b
Should show unknown and b removed
$ hg status
R b
? unknown
Should show a and unknown
$ ls
a
unknown