parser: Include extended header lines in diff

parser: Include extended header lines in diff

Commit 753e4572d updated the parser to consider additional header lines
when deciding where a patch message ends and the diff begins. However,
these additional lines were not captured meaning these patches didn't
have a diff associated with them and they therefore weren't patches in
the Patchwork sense of the term. Correct this and add a test.

Signed-off-by: Stephen Finucane <stephen@that.guru>
(cherry picked from commit fc1d7505991474b07638a87c0834157d12e1e476)