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.

Command/Export.hs

@@ -136,6 +136,7 @@ changeExport r db (PreferredFiltered new) = do
 			(Git.DiffTree.file diff)
 	forM_ (incompleteExportedTreeishes old) $ \incomplete ->
 		mapdiff recover incomplete new
+	waitForAllRunningCommandActions
 
 	-- Diff the old and new trees, and delete or rename to new name all
 	-- changed files in the export. After this, every file that remains
@@ -158,12 +159,14 @@ changeExport r db (PreferredFiltered new) = do
 					(Just oldf, Nothing) ->
 						startUnexport' r db oldf ek
 					_ -> stop
+			waitForAllRunningCommandActions
 			-- Rename from temp to new files.
 			seekdiffmap $ \(ek, (moldf, mnewf)) ->
 				case (moldf, mnewf) of
 					(Just _oldf, Just newf) ->
 						startMoveFromTempName r db ek newf
 					_ -> stop
+			waitForAllRunningCommandActions
 		ts -> do
 			warning "Resolving export conflict.."
 			forM_ ts $ \oldtreesha -> do
@@ -181,6 +184,7 @@ changeExport r db (PreferredFiltered new) = do
 					(\diff -> commandAction $ startUnexport r db (Git.DiffTree.file diff) (unexportboth diff))
 					oldtreesha new
 			updateExportTree db emptyTree new
+			waitForAllRunningCommandActions
 	liftIO $ recordExportTreeCurrent db new
 
 	-- Waiting until now to record the export guarantees that,
@@ -239,6 +243,7 @@ fillExport r db (PreferredFiltered newtree) mtbcommitsha = do
 	allfilledvar <- liftIO $ newMVar (AllFilled True)
 	commandActions $ map (startExport r db cvar allfilledvar) l
 	void $ liftIO $ cleanup
+	waitForAllRunningCommandActions
 
 	case mtbcommitsha of
 		Nothing -> noop
@@ -484,3 +489,4 @@ filterPreferredContent r tree = logExportExcluded (uuid r) $ \logwriter -> do
 					)
 			-- Always export non-annexed files.
 			Nothing -> return (Just ti)
+