fastannotate: adapt to buffer() going a way in Python 3
There's probably something more efficient I could do here, but I'm disinclined
to spend much time on this at the moment.
Differential Revision: https://phab.mercurial-scm.org/D5614
fastannotate: slice strings to get single character
Behaves identically on Python 3 and Python 2.
Differential Revision: https://phab.mercurial-scm.org/D5612