type based git config handling

Now there's a Config type, that's extracted from the git config at startup.
Note that laziness means that individual config values are only looked up
and parsed on demand, and so we get implicit memoization for all of them.
So this is not only prettier and more type safe, it optimises several
places that didn't have explicit memoization before. As well as getting rid
of the ugly explicit memoization code.

Not yet done for annex.<remote>.* configuration settings.

Annex/Ssh.hs

@@ -18,9 +18,8 @@ import Common.Annex
 import Annex.LockPool
 import Annex.Perms
 #ifndef WITH_OLD_SSH
-import qualified Git.Config
-import Config
 import qualified Build.SysConfig as SysConfig
+import qualified Annex
 #endif
 
 {- Generates parameters to ssh to a given host (or user@host) on a given
@@ -60,8 +59,7 @@ sshInfo (host, port) = ifM caching
 	caching = return False
 #else
 	caching = fromMaybe SysConfig.sshconnectioncaching 
-		. Git.Config.isTrue
-		<$> getConfig (annexConfig "sshcaching") ""
+		. annexSshCaching <$> Annex.getConfig
 #endif
 
 cacheParams :: FilePath -> [CommandParam]