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.

Git/ConfigTypes.hs

@@ -5,9 +5,12 @@
  - Licensed under the GNU AGPL version 3 or higher.
  -}
 
+{-# LANGUAGE OverloadedStrings #-}
+
 module Git.ConfigTypes where
 
 import Data.Char
+import qualified Data.ByteString.Char8 as S8
 
 import Common
 import Git
@@ -18,7 +21,7 @@ data SharedRepository = UnShared | GroupShared | AllShared | UmaskShared Int
 
 getSharedRepository :: Repo -> SharedRepository
 getSharedRepository r =
-	case map toLower $ Git.Config.get "core.sharedrepository" "" r of
+	case S8.map toLower $ Git.Config.get "core.sharedrepository" "" r of
 		"1" -> GroupShared
 		"2" -> AllShared
 		"group" -> GroupShared
@@ -26,14 +29,14 @@ getSharedRepository r =
 		"all" -> AllShared
 		"world" -> AllShared
 		"everybody" -> AllShared
-		v -> maybe UnShared UmaskShared (readish v)
+		v -> maybe UnShared UmaskShared (readish (decodeBS' v))
 
 data DenyCurrentBranch = UpdateInstead | RefusePush | WarnPush | IgnorePush
 	deriving (Eq)
 
 getDenyCurrentBranch :: Repo -> DenyCurrentBranch
 getDenyCurrentBranch r =
-	case map toLower $ Git.Config.get "receive.denycurrentbranch" "" r of
+	case S8.map toLower $ Git.Config.get "receive.denycurrentbranch" "" r of
 		"updateinstead" -> UpdateInstead
 		"warn" -> WarnPush
 		"ignore" -> IgnorePush