compare actual files rather than file paths

ran into an issue where `delete_original = True` and `convert-mp4 = True`, when the path given to the script is a relative path.

Currently, if the file is `/foo/bar/baz/`, and the path given is `./baz` inside of the `/foo/bar` path, the check is essentially `./baz == /foo/bar/baz`, which obviously fails, and as a result, the output file is "created" which overwrites the input, and then everything gets screwy.