finish is not always called in LCS algorithm

[madsmtm]

The LCS implementation does not always call finish at the end of its execution. This can cause issues when used with diff hooks like Replace.

You can use the code below as a regression test.

use similar::algorithms::{lcs, DiffHook};

struct HasRunFinish(bool);

impl DiffHook for HasRunFinish {
    type Error = ();
    fn finish(&mut self) -> Result<(), Self::Error> {
        self.0 = true;
        Ok(())
    }
}

let mut d = HasRunFinish(false);
let slice = &[1, 2];
lcs::diff(&mut d, slice, 0..slice.len(), slice, 0..slice.len()).unwrap();
assert!(d.0);