Make hunking block for consistent subsequent actions

[kaste]

We must ensure that hitting 'h' very fast does run completely ordered, and
always waits for 'gs_diff_refresh' to complete. ('gs_diff_refresh' updates our
model.)

Please NOTE that running a command from the worker will still run that command
on the main thread. T.i. set_timeout_async(lambda: view.run_command("refresh"))
is misleading and a footgun because the "refresh" command here will still start
on the main thread. We basically just await the worker and then put a
task/command on the main worker queue.

Fixes #1104

[randy3k]

I don't know if it is better to use sync operations or a lock.

[kaste]

Sorry, don't know what you mean here. What are your concerns?

'Make hunking block' is maybe a bit short. What I actually do here is make the hunk action one task. Do not split it up into multiple tasks. That is so that you can put 'hhh' three hunking tasks on the queue, and each of them runs like a function from start to end.

It would be a misunderstanding that it is necessary to run these tasks on the main thread then. It is totally possible to run the hunking - each as one block/task - on the worker or any other thread. The reason to run it on the main thread is purely for UI/UX. It just looks and feels nicer if you do it on the main thread because we avoid intermediate redraws and yank.

[randy3k]

Ok, thank you for the explanations. I thought you wanted to wait for an action to be completed before triggering another h. And I was thinking that a lock may do a better job in this aspect.