test-hgrc: do not print ui.plain() flag at uisetup()
This makes the test pass with chg. Since uisetup() is run per process,
"hg showconfig" does not always call uisetup().
# 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