v5 for direct mode, with automatic upgrade

This includes storing the current state of the HEAD ref, which git annex
sync is going to need, but does not make sync use it.

Annex/Version.hs

@@ -19,18 +19,21 @@ defaultVersion :: Version
 defaultVersion = "3"
 
 directModeVersion :: Version
-directModeVersion = "4"
+directModeVersion = "5"
 
 supportedVersions :: [Version]
 supportedVersions = [defaultVersion, directModeVersion]
 
 upgradableVersions :: [Version]
 #ifndef mingw32_HOST_OS
-upgradableVersions = ["0", "1", "2"]
+upgradableVersions = ["0", "1", "2", "4"]
 #else
-upgradableVersions = ["2"]
+upgradableVersions = ["2", "4"]
 #endif
 
+autoUpgradeableVersions :: [Version]
+autoUpgradeableVersions = ["4"]
+
 versionField :: ConfigKey
 versionField = annexConfig "version"
 
@@ -42,12 +45,3 @@ setVersion = setConfig versionField
 
 removeVersion :: Annex ()
 removeVersion = unsetConfig versionField
-
-checkVersion :: Version -> Annex ()
-checkVersion v
-	| v `elem` supportedVersions = noop
-	| v `elem` upgradableVersions = err "Upgrade this repository: git-annex upgrade"
-	| otherwise = err "Upgrade git-annex."
-  where
-	err msg = error $ "Repository version " ++ v ++
-		" is not supported. " ++ msg