[abhinav]

A prepared item and its dependents can disappear from the queue without
a completion, failure, or skip outcome.
The single prepare worker can accept another item as soon as it buffers
a result, before the scheduler consumes that result.
Consuming the older result then clears the preparing flag for the newer
call, allowing the scheduler to return while preparation is still running.

Keep preparation admission closed until its result has been consumed.
Serializing worker calls alone does not preserve that accounting;
the scheduler must acknowledge each call before dispatching the next.

[abhinav]

This change is part of the following stack:

• submit: Preserve scope in native stack updates #1486
  • merge: Separate execution responsibilities #1484
    • mergequeue: Await preparation results #1488 ◀
      • merge: Use native ranges when available #1401
        • docs: Explain native GitHub stacks #1406

_{Change managed by git-spice.}