tests/test-gendoc-fr.t

tests: avoid using rebaseskipobsolete=0

I'm about to delete the `rebaseskipobsolete` config. This patch
updates a test to get hidden commits into the rebase state by using
`hg debugobsolete` instead of setting `rebaseskipobsolete=0`.

Differential Revision: https://phab.mercurial-scm.org/D10270

#require docutils gettext

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