test-bundle2: also test the argument of the changegroup hook We also track execution of the changegroup hook. The important information here is to make sure the information that the transaction was processing a bundle2 is passed to hook. This will let most hooks disable themselves while waiting for the hook concluding bundle2 processing (the one we discovered to be not called for pull in the previous changesets).

test-bundle2: test that we got appropriate hook called with appropriate data We can notice that this transaction wide hook is only happening during push and it is missing changegroup-related information. We'll want to fix this but this is not what this patch is about.

pull: use `stepsdone` instead of `todosteps` The push process uses a `stepsdone` attribute instead of a `todosteps` one (with the logic swapped). We unify the two process by picking the `stepsdone` version. I feel like `stepsdone` better fits extensions that would want to extend the push exchange process.

pull: make discovery phase extensible We apply the same approach as for push and make the discovery extensible. There is only one user in core right now, but we already know we'll need something smarter for obsmarkers. In fact the evolve extension could use this to cleanly extend discovery. The main motivation for this change is consistency between push and pull.

sshpeer: forward stdout of remote "hg init" to appropriate output channel Otherwise, commandserver channel could be corrupted.

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).