Add groupcompress-block[-ref] as valid stream types.
However, we now encounter that the versionedfile tests assume that Factory objects know the content's sha1 sum if we request them with delta_closure=False. I assume this is because we had been reading the knit content headers to get the sha1sum, but I think Andrew was planning on changing that anyway. The VF tests expose the lack of sha1, but otherwise things seem to work if we pass back the lazy block, rather than extracting it ahead of time.