--cleanup --only-new deletes previously exported files

[RhetTbull]

osxphotos export --cleanup --only-new will delete previously exported files that weren't part of the --only-new export set. It would be better to read the list of previously exported files from the export database and include these in the keep set. This might be a different result than --cleanup by itself but is closer to what the user expects and it's better to leave extra files in place than delete everything.

See also #1505