More auto-cleanup of `virtual-branches.toml` when project is loaded.

[Byron]

Note that this code must not run when we write the file via RefMetadata. Instead, I think it's most time-efficient to put the 'fixup' code into the writer, and let it bail if RUST_BACKTRACE is enabled and the fixed up version of the data doesn't match the one we'd want to write.
That way, we can detect faulty writers on demand and maybe find it locally.

Lastly, the fixup code should run when a project is set active to fix it, giving code that needs it a fighting chance.

Tasks

• impl
• double-check that dlib-rs is not showing issues anymore after it was fixed: dlib-rs.zip
• ensure 'small' is working on master

Apparently also causes this:

thread 'tokio-runtime-worker' (5125663) panicked at crates/but-meta/src/legacy.rs:571:22:
we just looked it up
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[Byron]

On this repository, in Nightly:

small.zip

It really does manage to show three stacks in single-branch mode, in the workspace projection!