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.

changegroup.getsubset: support multiple versions Allow a version parameter to specify which version of the packer should be used