The response of a request is cached and it is invalidated every time one
of then packages of the new request is involve ind the differencies
between the new opam commit and the old one(cached opam commit).

Anytime the cached response is the same as the new response, we keep the
old one for the oldest opam commits which are used.

Invalidate the cache when the new commit is older in the commit history.

On addition to direct dependencies some transitive deps could also
change between 2 commits of opam repository.

* Same solution with different commit (is_same_solution).
* Same commits have no changes.