test: use a more verbose output in the test
While debugging some test failure, I released the test never checks if the
relevant changesets were preserved. So I am updating the test from `hg parents`
usage to `hg log -G` with a special template. This increase the area covered by
the test and clarify the test failures.
#require docutils gettext
$ $TESTDIR/check-gendoc el
checking for parse errors