Improve bash completion, so it completes names of remotes and backends in appropriate places.

Not necessarily everywhere, but a lot of the most often used places.

Re the use of .Internal, see
https://github.com/pcapriotti/optparse-applicative/issues/155

Command/Sync.hs

@@ -70,7 +70,10 @@ data SyncOptions  = SyncOptions
 
 optParser :: CmdParamsDesc -> Parser SyncOptions
 optParser desc = SyncOptions
-	<$> cmdParams desc
+	<$> (many $ argument str
+		( metavar desc
+		<> completeRemotes
+		))
 	<*> invertableSwitch "commit" True
 		( help "avoid git commit" 
 		)