fix hang if external program is not available

startExternal' throws an exception, which left the externalAsync TMVar
empty, so the next try to use it would hang.

Remote/External.hs

@@ -616,6 +616,7 @@ startExternal external =
 	liftIO (atomically $ takeTMVar (externalAsync external)) >>= \case
 		UncheckedExternalAsync -> do
 			(st, extensions) <- startExternal' external
+				`onException` store UncheckedExternalAsync
 			if asyncExtensionEnabled extensions
 				then do
 					relay <- liftIO $ runRelayToExternalAsync external st

doc/bugs/testremote_with_external_remote_hangs_after_asyncexternal_merge.mdwn

@@ -56,3 +56,5 @@ With the first commit before the merge checked out (0900bb2ca), the
 
 [[!meta author=kyle]]
 [[!tag projects/datalad]]
+
+> [[fixed|done]] thanks for reporting --[[Joey]]