transaction: pass the name of the "undo" journal to the transaction It is time for the transaction to be responsible for setting up the undo data. It is necessary to move this logic into the transaction because many more files are handled now, and the transaction is the object tracking them all. The value can be set to None if no undo should be set.

rollback: have an empty entry for the vfsmap in rollback This empty string key is used for the store. This will be needed to properly rollback backup in a future changesets.

transaction: clarify the name of 'journal' argument for transaction The argument is a string containing the journal name (used as prefix for all other transaction file). This is not the transaction file itself. So we clarify this.

transaction: use 'util.copyfile' for creating backup Using 'copyfile' (single file) instead of 'copyfiles' (tree) will ensures destination file will be overwritten. This will prevent some abort if backup file are left in place for random reason. It also seems more correct.

copyfile: allow optional hardlinking Some code paths use 'copyfiles' (full tree) for a single file to take advantage of the best-effort-hard-linking parameter. We add similar parameter and logic to 'copyfile' (single file) for this purpose. The single file version have the advantage to overwrite the destination file if it exists.

repair: add experimental option to write bundle2 files This adds an experimental option 'strip-bundle2-version' which causes backup bundles to use bundle2 formatting. Especially for generaldelta repositories, this should provide significant performance gains for any operation that needs to write a backup.