largefiles: update comments to refer to the right overridden method This cleans up leftovers from b228ad1f79d7 (largefiles: override calculateupdates instead of manifestmerge, 2014-03-02).

revlog: cache chain info after calculating it for a rev (issue4452) This dumb cache works surprisingly well: on a repository with typical delta chains ~50k in length, unbundling a linear series of 5000 revisions (changelogs and manifests only) went from 60 seconds to 3.

manifest: add matches() method Move the code in context._manifestmatches() into a new manifest.matches(). It's a natural place for the code to live and it allows other callers to easily use it. It should also make it easier to optimize the new method in alternative implementations of the manifest (same reasoning as with manifest.diff()).