config: fix py3 backslash escaping bug in parser caught by tests
Differential Revision: https://phab.mercurial-scm.org/D3525
# The 9diff extdiff command generates diff(1) compatible output
# suitable for use with the plumber(4).
[extensions]
extdiff =
[extdiff]
9diff = /bin/mercurial/9diff -cm $parent $child $root