Fix parsing of interesing series reply structures

Fix parsing of interesing series reply structures

There are some things you probably shouldn't do on public
mailing lists, but which people do anyway.

The first, and most understandable, is this:

          - [PATCH 1/2] test: Add some lorem ipsum
            - [PATCH 2/2] test: Convert to Markdown
              - [PATCH v2 1/2] test: Add some lorem ipsum
                - [PATCH v2 2/2] test: Convert to Markdown

We should correctly parse these by:
 - creating a new series if the version number changes
 - when deciding whether to create a SeriesReference, search by
   message-id alone, not the message-id/series pair. (Otherwise,
   we try to create a series ref for v1 2/2 in the series for v2,
   which breaks a uniqueness constraint.

The second, and less excusable, is this:

          - [PATCH 1/2] test: Add some lorem ipsum
            - [PATCH 2/2] test: Convert to Markdown
              - [PATCH 1/2] test: Add some lorem ipsum
                - [PATCH 2/2] test: Convert to Markdown

With this patch:
 - if we get a x/n for a series that already has an x/n, create a
   new series for it.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Stephen Finucane <stephen@that.guru>