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.

Git/LsTree.hs

@@ -19,8 +19,6 @@ import qualified Data.ByteString.Lazy.Char8 as L
 import Git
 import Utility.SafeCommand
 
-type Treeish = String
-
 data TreeItem = TreeItem
 	{ mode :: FileMode
 	, typeobj :: String
@@ -28,10 +26,10 @@ data TreeItem = TreeItem
 	, file :: FilePath
 	} deriving Show
 
-{- Lists the contents of a Treeish -}
-lsTree :: Treeish -> Repo -> IO [TreeItem]
+{- Lists the contents of a Ref -}
+lsTree :: Ref -> Repo -> IO [TreeItem]
 lsTree t repo = map parseLsTree <$>
-	pipeNullSplitB [Params "ls-tree --full-tree -z -r --", File t] repo
+	pipeNullSplitB [Params "ls-tree --full-tree -z -r --", File $ show t] repo
 
 {- Parses a line of ls-tree output.
  - (The --long format is not currently supported.) -}