revlog: add a fast method for getting a list of chunks This moves _chunkraw into the loop. Doing that improves revlog decompression -- in particular, manifest decompression -- significantly. For a 20 MB manifest which is the result of a > 40k delta chain, hg perfmanifest improves from 0.55 seconds to 0.49 seconds.

perfmanifest: allow and require passing in a rev Previously perfmanifest would only test tip.