Revert "Avoid duplicate code by having `zip_archive()` use the `get_included_files()` method"

This reverts commit 8ad2ac5b25c504a8dcbcabf21495170759990a00.

Memory issues: https://github.com/humanmade/hm-backup/issues/45#issuecomment-26817146
Use `get_files()`, which returns an iterator, which will be more performant for large sets of files
Boost performance for large backups
Support for file manifest with ZipArchive