edit/prompt: Fix a case where the prompt should not be marked stale.
edit/prompt: Fix a case where the prompt should not be marked stale.

Previously, when the prompt callback finishes and there is already
another queued update, the prompt is immediately marked as stale. This
is now done only if the prompt callback didn't finish within the
threshold and was already marked as stale.

This fixes #630.

elves/elvish,elves/elvish,elves/elvish,elves/elvish,elves/elvish