changegroup: differentiate between fulltext and diff based deltas Previously, revisiondelta encoded a delta and an optional prefix containing a delta header. The underlying code could populate the delta with either a real delta or a fulltext revision. Following the theme of wanting to defer serialization of revision data to the changegroup format as long as possible, it seems prudent for the revision delta instance to capture what type of data is being represented. This could possibly allow us to encode revision data differently in the future. But for the short term, it makes the behavior of a revisiondelta more explicit. Differential Revision: https://phab.mercurial-scm.org/D4213