tests: make autodiff.py work on Python 3
This patch adds b'' to string literal to make it compatible with in-core Python
3 compatibility.
# skip-blame because we are just adding b''
Differential Revision: https://phab.mercurial-scm.org/D1791
$ 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