When inserting patch sets, pass in a RevWalk along with the commit
SHA-1 so we can ensure the body is parsed. Set the subject implicitly
when we add a new patch set; always do this, even if the subject
matches the previous subject.
When parsing patch sets, we have to still parse the subject separately
from the patch set, since there is no subject field on PatchSet. But
we can use a single setter on Change to set the current patch set ID,
subject, and original subject together as a batch, rather than going
through PatchSetInfo.
Change-Id: If37c4990e2e5888d5e87bd5c34a821e59186ab6d
5dd9c1ac98