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