revert: drop now useless conditional in the backup check Now that we removed the (hopeless) attempt to backup file we knew to be missing in the target changeset, we can stop checking if the file exists in the target changeset.

revert: no backup for `dsadded` set There is only one case where a backup is required in the `dsadded` set, and the current backup mechanism fails to handle it. So we stop trying to do backups at all for now. This will help us to simplify the backup code and finally fix this backup issue.

revert: add more padding in the dispatch list We are going to add more sets and some of them will have longer names. We add padding in a standalone patch for readbility purposes.

revert: add documentation in the dispatch table More sets are coming so documenting the existing ones will help.

revert: add a way for external extensions to prefetch file data This allow extensions that mess with the storage layer (remotefilelog, largefile) to prefetch any data that will be accessed during the revert operation. We are currently fetching more data than theoretically required because the backup code is a bit stupid. Future patches will improve that.

merge with stable