on-save-file (augments, and augmentable only) in editor:backup-autosave-mixin

Called just before the editor is saved to a file, after calling can-save-file? to verify that the save is allowed. See also after-save-file.

• (send an-editor:backup-autosave-mixin on-save-file filename format bool
     filename : path
     format : symbol in '(guess standard text text-force-cr same copy)

  If a backup file has not been created this session for this file, deletes any existing backup file and copies the old save file into the backup file. For the backup file's name, see path-utils:generate-backup-name