sync: Fix committing when in a direct mode repo that has no HEAD ref.

Seen for example, a newly checked out git submodule. In this case,
.git/HEAD is a raw sha, rather than the usual reference to a ref.

Removed currentSha in passing, since it was a more roundabout way of
doing what headSha does, and headSha is more robust.

Command/Proxy.hs

@@ -13,8 +13,8 @@ import Config
 import Utility.Tmp
 import Utility.Env
 import Annex.Direct
-import qualified Git.Branch
 import qualified Git.Sha
+import qualified Git.Ref
 
 cmd :: [Command]
 cmd = [notBareRepo $
@@ -35,7 +35,7 @@ start (c:ps) = liftIO . exitWith =<< ifM isDirect
   where
 	go tmp = do
 		oldref <- fromMaybe Git.Sha.emptyTree
-			<$> inRepo Git.Branch.currentSha
+			<$> inRepo Git.Ref.headSha
 		exitcode <- liftIO $ proxy tmp
 		mergeDirectCleanup tmp oldref
 		return exitcode