--update/--update-all not working in some edge cases

[sbidoul]

When a new dependency is added to the project, and it is already installed while not being part of requirements.txt, using --update or --update-all may not update it to the latest available version.

This is a very edge case that is probably not an issue in practice.

[sbidoul]

To fix this we can compute the following set after pip-df-sync: ( {installed before} \ {installed dependencies before} ) ∩ { installed dependencies after }. If it is not empty, run a second pass.

Alternatively, I should dig into pip upgrade strategies and make sure they work for all our use cases (direct URLs, etc).