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 <>
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