deleting the last branch in a project makes things awkward

[ceedubs]

If you have only one branch in a project (let's say project foo and branch main) and you delete it, it's unclear how to continue to use the project.

If you try switch foo then it says that foo/main doesn't exist.

If you try branch foo/main then it will create main from your current namespace, which is likely the root of your codebase (and in my case takes ages to subsequently delete.branch).

.> project.create foo

  🎉 I've created the project foo.

  I'll now fetch the latest version of the base Unison library...
updateRoot: 531 ms (cpu), 548 ms (system)

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InputPattern: 619 ms (cpu), 1.78 s (system)
foo/main> branch.delete main
updateRoot: 259 ms (cpu), 270 ms (system)
InputPattern: 265 ms (cpu), 299 ms (system)
.> switch foo

  foo/main does not exist.
InputPattern: 1.82 ms (cpu), 4.56 ms (system)
.> branch foo/main
updateRoot: 16.3 ms (cpu), 27.4 ms (system)

  Done. I've created the foo/main branch from the namespace ..
InputPattern: 21.0 ms (cpu), 55.8 ms (system)