improve git command queue flushing with time limit

So that eg, addurl of several large files that take time to download will
update the index for each file, rather than deferring the index updates to
the end.

In cases like an add of many smallish files, where a new file is being
added every few seconds. In that case, the queue will still build up a
lot of changes which are flushed at once, for best performance. Since
the default queue size is 10240, often it only gets flushed once at the
end, same as before. (Notice that updateQueue updated _lastchanged
when adding a new item to the queue without flushing it; that is
necessary to avoid it flushing the queue every 5 minutes in this case.)

But, when it takes more than a 5 minutes to add a file, the overhead of
updating the index immediately is probably small, so do it after each
file. This avoids git-annex potentially taking a very very long time
indeed to stage newly added files, which can be annoying to the user who
would like to get on with doing something with the files it's already
added, eg using git mv to rename them to a better name.

This is only likely to cause a problem if it takes say, 30 seconds to
update the index; doing an extra 30 seconds of work after every 5
minute file add would be less optimal. Normally, updating the index takes
significantly less time than that. On a SSD with 100k files it takes
less than 1 second, and the index write time is bound by disk read and
write so is not too much worse on a hard drive. So I hope this will not
impact users, although if it does turn out to, the time limit could be
made configurable.

A perhaps better way to do it would be to have a background worker
thread that wakes up every 60 seconds or so and flushes the queue.
That is made somewhat difficult because the queue can contain Annex
actions and so this would add a new source of concurrency issues.
So I'm trying to avoid that approach if possible.

Sponsored-by: Erik Bjäreholt on Patreon

Annex/Queue.hs

@@ -78,7 +78,8 @@ get = maybe new return =<< getState repoqueue
 
 new :: Annex (Git.Queue.Queue Annex)
 new = do
-	q <- Git.Queue.new . annexQueueSize <$> getGitConfig
+	sz <- annexQueueSize <$> getGitConfig
+	q <- liftIO $ Git.Queue.new sz Nothing
 	store q
 	return q