parsemail: Handle cover letters sent in reply

parsemail: Handle cover letters sent in reply

Some people send cover letters (and therefore series) as replies to
existing series. Apply a second set of heuristics to cover this case:

* The message contains a '[0/n]' marker tag in the subject
* The message is not the root message, but the subject is unique, i.e.
  it is not a reply to a cover letter already stored in Patchwork

It is theoretically possible that the message could be a reply to a
cover letter that's been missed, but such occurences should be rare
and only get rarer the longer an instance is running.

Note that there is a third case which is not yet covered: a cover
letter with the same name but a different number of patches. This
will have to wait until series support is fully implemented.

Signed-off-by: Stephen Finucane <stephen.finucane@intel.com>
Reviewed-by: Andy Doan <andy.doan@linaro.org>