tests/test-gendoc-it.t

templater: provide hint for multi-line templates with parse errors

Previously, we punted. Now we "rewrite" the template's newlines to r'\n' and
offset the hint appropriately.

Differential Revision: https://phab.mercurial-scm.org/D2609

#require docutils gettext

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