ordered_set: operator<: Fix for a case of strict inclusion.

[DoctorNoobingstoneIPresume]

When all elements compared so far in sets x and y have been equivalent
and one of the sets (x) ends, but the other one (y) does not end
(i.e. x is a strict prefix of y),
we used to get y < x, but with this changeset we are going to get x < y.

Example (in pseudocode) (with case-insensitive comparison):
we used to get {'A', 'b'} > {'a', 'B', 'c'},
now fixed to {'A', 'b'} < {'a', 'B', 'c'}.
This is similar to how std::lexicographical_compare behaves.

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[DoctorNoobingstoneIPresume]

Perhaps we could replace the whole implementation (containing the loop) with just:

return std::lexicographical_compare
(
      data_map.begin (),   data_map.end (),
    a.data_map.begin (), a.data_map.end (),
    data_map.key_comp ()
);

[mihaibudiu]

Can you please try to run make cpplint prior to submitting the PR?

[DoctorNoobingstoneIPresume]

Can you please try to run make cpplint prior to submitting the PR?

Thank you for review. I will try to do that. I dare suggest that maybe we remove cpplint ? :)

E.g.: Alternative implementation with std::lexicographical_compare (in comment above: #3431 (comment)) is never going to pass cpplint as it is, but I think it is clearer -- the additional whitespace makes obvious what is similar and what is different between the two lines.

[mihaibudiu]

Having tools makes it much easier to enforce coding standards. Without tools like cpplint conversations can devolve into religious arguments. For example, I hate the defaults of tools like rustfmt, but I save a lot of time avoiding arguing about the right way to format Rust. In the end, time is more valuable.