Due to complex handling of incoming series all the new cover letters
were not used.

parsemail creates new, transient revision that is later discarded.
Everything that should get persistent in the proper revision has to be
copied over explicitly in on_revision_complete signal handler.

So let's do that for all the cover_letter fields, but only if they are
present.

Signed-off-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>