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/Drop.hs

@@ -46,6 +46,7 @@ parseDropFromOption :: Parser (DeferredParse Remote)
 parseDropFromOption = parseRemoteOption $ strOption
 	( long "from" <> short 'f' <> metavar paramRemote
 	<> help "drop content from a remote"
+	<> completeRemotes
 	)
 
 seek :: DropOptions -> CommandSeek

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)

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" 
 		)