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.

Git/Ref.hs

@@ -88,6 +88,9 @@ sha branch repo = process <$> showref repo
 	process [] = Nothing
 	process s = Just $ Ref $ firstLine s
 
+headSha :: Repo -> IO (Maybe Sha)
+headSha = sha headRef
+
 {- List of (shas, branches) matching a given ref or refs. -}
 matching :: [Ref] -> Repo -> IO [(Sha, Branch)]
 matching refs repo =  matching' (map fromRef refs) repo