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
pager.pager=p0 # $TESTTMP/sysrc:4
ui.editor=e1 # $TESTTMP/userrc:2
pager.pager=p2 # $PAGER
ui.editor=e1 # $TESTTMP/userrc:2