Release version 12

* NEWS: Record release date.
maint: post-release administrivia.

* NEWS: Add header line for next release.
* .prev-version: Record previous version.
* ./local.mk (old_NEWS_hash): Auto-update.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
formatters: don't elide repeated nested contexts.

* specs/formatter/report_spec.yaml
(with repeated description elements): Specify the output when
different yaml branches share an element description.
* lib/specl/formatter/report.lua (show_contexts): Adjust to
match specified behaviour.
* NEWS: Update.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>