tests/test-gendoc-da.t

diff: replace --merge option by config option

I can't think of any reason you'd want to enable the merge diff on a
run-to-run basis; you'd probably either always or never want it set
(though I can't see why you'd never want it set). If you have it set,
you'll probably also want the same output in `hg log -p`
output. Having a single config option for the feature makes sense.

Differential Revision: https://phab.mercurial-scm.org/D9956

#require docutils gettext

  $ $TESTDIR/check-gendoc da
  checking for parse errors