[MERGE] Fix trivial bug in an AssertionError.

[Andrew Bennetts]

Andrew Bennetts wrote:
> This bug was found in the course of diagnosing another bug
> (https://bugs.launchpad.net/bzr/+bug/252445).  I wish pyflakes would detect
> these!  Hmm, pychecker does detect them...

FWIW, I hacked pyflakes to detect this problem
(lp:~spiv/pyflakes/detect-broken-string-formatting), and didn't find any other
instances of this bug in bzr.dev. :)

-Andrew.