external special remotes mostly implemented (untested)

This has not been tested at all. It compiles!

The only known missing things are support for encryption, and for get/set
of special remote configuration, and of key state. (The latter needs
separate work to add a new per-key log file to store that state.)

Only thing I don't much like is that initremote needs to be passed both
type=external and externaltype=foo. It would be better to have just
type=foo

Most of this is quite straightforward code, that largely wrote itself given
the types. The only tricky parts were:

* Need to lock the remote when using it to eg make a request, because
  in theory git-annex could have multiple threads that each try to use
  a remote at the same time. I don't think that git-annex ever does
  that currently, but better safe than sorry.

* Rather than starting up every external special remote program when
  git-annex starts, they are started only on demand, when first used.
  This will avoid slowdown, especially when running fast git-annex query
  commands. Once started, they keep running until git-annex stops, currently,
  which may not be ideal, but it's hard to know a better time to stop them.

* Bit of a chicken and egg problem with caching the cost of the remote,
  because setting annex-cost in the git config needs the remote to already
  be set up. Managed to finesse that.

This commit was sponsored by Lukas Anzinger.

Config.hs

@@ -12,7 +12,6 @@ import qualified Git
 import qualified Git.Config
 import qualified Git.Command
 import qualified Annex
-import qualified Types.Remote as Remote
 import Config.Cost
 
 type UnqualifiedConfigKey = String
@@ -55,14 +54,16 @@ annexConfig key = ConfigKey $ "annex." ++ key
  - by remote.<name>.annex-cost, or if remote.<name>.annex-cost-command
  - is set and prints a number, that is used. -}
 remoteCost :: RemoteGitConfig -> Cost -> Annex Cost
-remoteCost c def = case remoteAnnexCostCommand c of
-	Just cmd | not (null cmd) -> liftIO $
-		(fromMaybe def . readish) <$>
-			readProcess "sh" ["-c", cmd]
-	_ -> return $ fromMaybe def $ remoteAnnexCost c
+remoteCost c def = fromMaybe def <$> remoteCost' c
 
-setRemoteCost :: Remote -> Cost -> Annex ()
-setRemoteCost r c = setConfig (remoteConfig (Remote.repo r) "cost") (show c)
+remoteCost' :: RemoteGitConfig -> Annex (Maybe Cost)
+remoteCost' c = case remoteAnnexCostCommand c of
+	Just cmd | not (null cmd) -> liftIO $
+		readish <$> readProcess "sh" ["-c", cmd]
+	_ -> return $ remoteAnnexCost c
+
+setRemoteCost :: Git.Repo -> Cost -> Annex ()
+setRemoteCost r c = setConfig (remoteConfig r "cost") (show c)
 
 getNumCopies :: Maybe Int -> Annex Int
 getNumCopies (Just v) = return v