subrepo: add status support for ignored and clean files in git subrepos

revlog: addgroup checks if incoming deltas add censored revs, sets flag bit A censored revision stored in a revlog should have the censored revlog index flag bit set. This implies we must know if a revision is censored before we add it to the revlog. When adding revisions from exchanged deltas, we would prefer to determine this flag without decoding every single full text. This change introduces a heuristic based on assumptions around the Mercurial delta format and filelog metadata. Since deltas which produce a censored revision must be full-replacement deltas, we can read the delta's first bytes to check the filelog metadata. Since "censored" is the alphabetically first filelog metadata key, censored filelog revisions have a well-known prefix we can look for. For more on the design and background of the censorship feature, see: http://mercurial.selenic.com/wiki/CensorPlan

patch.internalpatch: accept a prefix parameter