disable buggy concurrency in Command.Export

Fix a crash or potentially not all files being exported when sync -J
--content is used with an export remote.

Crash as described in fixed bug report.

waitForAllRunningCommandActions inserted in several points where all the
commandActions started before need to have finished before moving on to
the next stage of the export. A race across those points could have
maybe resulted in not all files being exported, or a wrong tree being
export.

For example, changeExport starting up an action like
a rename of A to B. Then, with that action still running, fillExport
uploading a new A, *before* the rename occurred. That race seems
unlikely to have happened. There are some other ones that this also
fixes.

CmdLine/Action.hs

@@ -1,6 +1,6 @@
 {- git-annex command-line actions and concurrency
  -
- - Copyright 2010-2019 Joey Hess <id@joeyh.name>
+ - Copyright 2010-2020 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU AGPL version 3 or higher.
  -}
@@ -136,6 +136,15 @@ finishCommandActions = Annex.getState Annex.workers >>= \case
 				else retry
 		mapM_ mergeState sts
 
+{- Waits for all worker threads that have been started so far to finish. -}
+waitForAllRunningCommandActions :: Annex ()
+waitForAllRunningCommandActions = Annex.getState Annex.workers >>= \case
+	Nothing -> noop
+	Just tv -> liftIO $ atomically $ do
+		pool <- readTMVar tv
+		unless (allIdle pool)
+			retry
+
 {- Like commandAction, but without the concurrency. -}
 includeCommandAction :: CommandStart -> CommandCleanup
 includeCommandAction start =