better dup key with -J fix

This avoids all the complication about redundant work discussed in
the previous try at fixing this. At the expense of needing each command
that could have the problem to be patched to simply wrap the action in
onlyActionOn once the key is known. But there do not seem to be many
such commands.

onlyActionOn' should not be used with a CommandStart (or CommandPerform),
although the types do allow it. onlyActionOn handles running the whole
CommandStart chain. I couldn't immediately see a way to avoid mistken
use of onlyActionOn'.

This commit was supported by the NSF-funded DataLad project.

Command/Get.hs

@@ -62,8 +62,8 @@ startKeys from key ai = checkFailedTransferDirection ai Download $
 	start' (return True) from key (AssociatedFile Nothing) ai
 
 start' :: Annex Bool -> Maybe Remote -> Key -> AssociatedFile -> ActionItem -> CommandStart
-start' expensivecheck from key afile ai = stopUnless (not <$> inAnnex key) $
-	stopUnless expensivecheck $
+start' expensivecheck from key afile ai = onlyActionOn key $
+	stopUnless (not <$> inAnnex key) $ stopUnless expensivecheck $
 		case from of
 			Nothing -> go $ perform key afile
 			Just src ->
@@ -109,10 +109,9 @@ getKey' key afile = dispatch
 		| Remote.hasKeyCheap r =
 			either (const False) id <$> Remote.hasKey r key
 		| otherwise = return True
-	docopy r = download (Remote.uuid r) key afile forwardRetry $ \p ->
-		ifM (inAnnex key)
-			( return True
-			, getViaTmp (RemoteVerify r) key $ \dest -> do
+	docopy r witness = getViaTmp (RemoteVerify r) key $ \dest ->
+		download (Remote.uuid r) key afile forwardRetry
+			(\p -> do
 				showAction $ "from " ++ Remote.name r
 				Remote.retrieveKeyFile r key afile dest p
-			)
+			) witness