The semantic of diff? is a bit flaky. It has been a problem for users in the past (#2396) and it doesn't play well with sandboxing. We were discussing it with @aalekseyev this morning and are proposing the following changes:

• the first argument will always be a required dependency (as discussed in Add a diff command asking for files whitout rule for their generation #2396)
• when the second argument exist, it will immediately be moved to a "promotion staging area" rather than be left where it was created

These two changes should make diff? better behaved, especially when sandboxing is enabled.

I'm putting this in the 2.0 milestone since it is a breaking change. We should have a quick at the dune-universe to check the impact of the first change. If as expected it is small, we should just make the change independently of the dune language version in use.