Fix all empty conflict messages

[kit-ty-kate]

This PR simplifies the construction of ct_chains. This has several effects:

• This fixes all the empty conflict messages described in Unhelpful conflict messages #4373
• This removes the hack added in Fix empty conflict explanations #4982
• This makes the conflict messages more verbose. This is not the greatest outcome but this is already what Unhelpful conflict messages #4373 is about so it will be fixed later (but empty conflict messages are the most important to fix first)

[rjbou]

thanks!