revlog: support importing censored file revision tombstones This change allows a revision log to not fail integrity checks when applying a changegroup delta (eg from a bundle) results in a censored file tombstone. The tombstone is inserted as-is, so future integrity verification will observe the tombstone. Deltas based on the tombstone will also remain correct. The new code path is encountered for *exactly* the cases where _addrevision is importing a tombstone from a changegroup. When committing a file containing the "magic" tombstone text, the "text" parameter will be non-empty and the checkhash call is not executed (and when committing, the node will be computed to match the "magic" tombstone text).

verify: report censored nodes if configured policy is abort

context: handle censored data in an on-disk file context based on config Two possible behaviors are defined for handling censored data: abort, and ignore. When we ignore censored data we return an empty file to callers requesting the file data.

manifest: add fastdelta method to manifestdict This is another step closer to alternate manifest implementations that can offer different hashing algorithms.