get -J: Download different files from different remotes when the remotes have the same costs.

Only done in -J mode because only if there's concurrency can downloading
from two remotes be faster. Without concurrency, it's likely the case that
sequential downloads from the same remote are faster than switching back
and forth between two remotes.

There is some hairy MVar code here, but basically it just keeps
the activeremotes MVar full except when deciding which remote to assign
to a thread.

Also affects gets by sync --content -J

This commit was sponsored by Jochen Bartl.

Command/Get.hs

@@ -89,16 +89,17 @@ getKey' key afile = dispatch
 		showNote "not available"
 		showlocs
 		return False
-	dispatch remotes = notifyTransfer Download afile $ trycopy remotes remotes
-	trycopy full [] _ = do
-		Remote.showTriedRemotes full
-		showlocs
-		return False
-	trycopy full (r:rs) witness =
-		ifM (probablyPresent r)
-			( docopy r witness <||> trycopy full rs witness
-			, trycopy full rs witness
+	dispatch remotes = notifyTransfer Download afile $ \witness -> do
+		ok <- pickRemote remotes $ \r -> ifM (probablyPresent r)
+			( docopy r witness
+			, return False
 			)
+		if ok
+			then return ok
+			else do
+				Remote.showTriedRemotes remotes
+				showlocs
+				return False
 	showlocs = Remote.showLocations False key []
 		"No other repository is known to contain the file."
 	-- This check is to avoid an ugly message if a remote is a