models: Copy cover letter when duplicating revision

Cover letter is also a part of revision "metadata". Coping it over makes
the overall experience better - all the meta tags included in the cover
are preserved and user can see it on revisions caused by a rerun or a
single patch bump.

The difference between duplicate_meta and duplicate is that duplicate
also copies over patches, this may need a better naming.

Signed-off-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
2 jobs for raw_cover in 8 minutes and 51 seconds (queued for 1 second)
Status Name Job ID Coverage
  Test
passed test #72211

00:08:51

failed test #72205

00:08:16