Fix the waiting status display for synchronous operations (#5659)

Commit 4f0393f97b06 caused a regression: for operations that use
WithWaitingStatusSync (examples are squashing fixups, moving commits up
or down, cherry-picking, creating fixup commits, and more), the waiting
status wouldn't show during the operation; however, it would show after
the operation was done, and then linger forever.

The cause: since 4f0393f97b06, layout sizes the bottom line from the
actual content of the AppStatus view rather than from the status
manager. The async render path keeps the view in sync (it sets the
buffer on the first tick and clears it to "" when the status ends), but
the sync path used by WithWaitingStatusSync did not:

- It called ForceLayoutAndRedraw before writing anything to the view, so
layout saw an empty buffer and left no room; the status never appeared
during the operation.
- When the operation finished it just broke out of the loop, leaving the
last spinner frame in the buffer. Every subsequent layout kept reserving
room for that stale content, so the status stuck around forever.

Fix this by writing the status into the view before the initial layout,
and clearing it again when stopping.