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/Fsck.hs

@@ -67,6 +67,7 @@ optParser desc = FsckOptions
 	<*> optional (parseRemoteOption $ strOption 
 		( long "from" <> short 'f' <> metavar paramRemote 
 		<> help "check remote"
+		<> completeRemotes
 		))
 	<*> optional parseincremental
 	<*> optional (parseKeyOptions False)