improve type signatures with a Ref newtype

In git, a Ref can be a Sha, or a Branch, or a Tag. I added type aliases for
those. Note that this does not prevent mixing up of eg, refs and branches
at the type level. Since git really doesn't care, except rare cases like
git update-ref, or git tag -d, that seems ok for now.

There's also a tree-ish, but let's just use Ref for it. A given Sha or Ref
may or may not be a tree-ish, depending on the object type, so there seems
no point in trying to represent it at the type level.

Command/Uninit.hs

@@ -26,9 +26,9 @@ check :: Annex ()
 check = do
 	b <- current_branch	
 	when (b == Annex.Branch.name) $ error $
-		"cannot uninit when the " ++ b ++ " branch is checked out"
+		"cannot uninit when the " ++ show b ++ " branch is checked out"
 	where
-		current_branch = head . lines . B.unpack <$> revhead
+		current_branch = Git.Ref . head . lines . B.unpack <$> revhead
 		revhead = inRepo $ Git.pipeRead 
 			[Params "rev-parse --abbrev-ref HEAD"]
 
@@ -57,5 +57,5 @@ cleanup = do
 	liftIO $ removeDirectoryRecursive annexdir
 	-- avoid normal shutdown
 	saveState
-	inRepo $ Git.run "branch" [Param "-D", Param Annex.Branch.name]
+	inRepo $ Git.run "branch" [Param "-D", Param $ show Annex.Branch.name]
 	liftIO exitSuccess