async-backup
Emacs has a built-in backup system, but it does not backup on each save. It can be made to, but that makes saving really slow (and the UI unresponsive), especially for large files.
Fortunately, Emacs has asynchronous processes.
To enable for all files -
(add-hook 'after-save-hook #'async-backup)
To enable for a specific file -
M-x add-file-local-variable RET eval RET (add-hook 'after-save-hook #'async-backup nil t) RET
License
Chronometrist is released under your choice of Unlicense or the WTFPL.