patchbomb: reorder header insertions to clarify code This is done as its own change to make the changes to test-patchbomb.t easier to review.

patchbomb: always use message-id of first patch for series-id This currently has the side effect that the 0 of N message has no series-id. This won't matter for Mercurial's own use, but may be undesirable for other projects depending on their workflow. The way the header is inserted is intentionally a little funny to make the test expectation diff easier to review.