Issuing a server request triggers `changetracking.flush` so as to
make sure we're not operating on a stale state. This immediately
triggers notification of any pending changes (as a result of debouncing)
to the server. However, this happens in addition to the notification
that is waiting on the debounce delay. Because we `nil`
`buf_state.pending_change` when it is called, the fix is to
also check that this is non-`nil` when it is called and exit if it is,
as this being `nil` would mean that it originates from a pending change
that has already been flushed out.
ac10c0f418
