In case of a mismatch, insert Left entries before Right

[utkarshkukreti]

These should pass:

test "#4" {
    assert_eq!(::diff::slice(&[1], &[2]), vec![::diff::Result::Left(&1),
                                               ::diff::Result::Right(&2)]);
    assert_eq!(::diff::lines("a", "b"), vec![::diff::Result::Left("a"),
                                             ::diff::Result::Right("b")]);
}

but they fail with:

thread 'speculate::_::bugs::#4' panicked at 'assertion failed: (left == right) (left: [Right(2), Left(1)], right: [Left(1), Right(2)])', tests/tests.rs:53

and

thread 'speculate::_::bugs::#4' panicked at 'assertion failed: (left == right) (left: [Right("b"), Left("a")], right: [Left("a"), Right("b")])', tests/tests.rs:53

This will probably fix rust-lang/rustfmt#606 too.