use ByteString for git config

The parser and looking up config keys in the map should both be faster
due to using ByteString.

I had hoped this would speed up startup time, but any improvement to
that was too small to measure. Seems worth keeping though.

Note that the parser breaks up the ByteString, but a config map ends up
pointing to the config as read, which is retained in memory until every
value from it is no longer used. This can change memory usage
patterns marginally, but won't affect git-annex.

Config.hs

@@ -6,6 +6,7 @@
  -}
 
 {-# LANGUAGE TypeSynonymInstances, FlexibleInstances #-}
+{-# LANGUAGE OverloadedStrings #-}
 
 module Config where
 
@@ -22,25 +23,31 @@ import qualified Types.Remote as Remote
 import qualified Annex.SpecialRemote.Config as SpecialRemote
 
 import qualified Data.Map as M
+import qualified Data.ByteString as S
 
-type UnqualifiedConfigKey = String
-data ConfigKey = ConfigKey String
+type UnqualifiedConfigKey = S.ByteString
+
+newtype ConfigKey = ConfigKey S.ByteString
 
 instance Show ConfigKey where
-	show (ConfigKey s) = s
+	show (ConfigKey s) = decodeBS' s
 
 {- Looks up a setting in git config. This is not as efficient as using the
  - GitConfig type. -}
-getConfig :: ConfigKey -> String -> Annex String
+getConfig :: ConfigKey -> S.ByteString -> Annex S.ByteString
 getConfig (ConfigKey key) d = fromRepo $ Git.Config.get key d
 
-getConfigMaybe :: ConfigKey -> Annex (Maybe String)
+getConfigMaybe :: ConfigKey -> Annex (Maybe S.ByteString)
 getConfigMaybe (ConfigKey key) = fromRepo $ Git.Config.getMaybe key
 
 {- Changes a git config setting in both internal state and .git/config -}
 setConfig :: ConfigKey -> String -> Annex ()
 setConfig (ConfigKey key) value = do
-	inRepo $ Git.Command.run [Param "config", Param key, Param value]
+	inRepo $ Git.Command.run
+		[ Param "config"
+		, Param (decodeBS' key)
+		, Param value
+		]
 	reloadConfig
 
 reloadConfig :: Annex ()
@@ -68,11 +75,11 @@ instance RemoteNameable Remote.RemoteConfig where
 {- A per-remote config setting in git config. -}
 remoteConfig :: RemoteNameable r => r -> UnqualifiedConfigKey -> ConfigKey
 remoteConfig r key = ConfigKey $
-	"remote." ++ getRemoteName r ++ ".annex-" ++ key
+	"remote." <> encodeBS' (getRemoteName r) <> ".annex-" <> key
 
 {- A global annex setting in git config. -}
 annexConfig :: UnqualifiedConfigKey -> ConfigKey
-annexConfig key = ConfigKey $ "annex." ++ key
+annexConfig key = ConfigKey ("annex." <> key)
 
 {- Calculates cost for a remote. Either the specific default, or as configured 
  - by remote.<name>.annex-cost, or if remote.<name>.annex-cost-command