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.hs

@@ -10,6 +10,10 @@
 
 module Git (
 	Repo,
+	Ref(..),
+	Branch,
+	Sha,
+	Tag,
 	repoFromCwd,
 	repoFromAbsPath,
 	repoFromUnknown,
@@ -94,6 +98,18 @@ data Repo = Repo {
 	remoteName :: Maybe String 
 } deriving (Show, Eq)
 
+{- A git ref. Can be a sha1, or a branch or tag name. -}
+newtype Ref = Ref String
+	deriving (Eq)
+
+instance Show Ref where
+	show (Ref v) = v
+
+{- Aliases for Ref. -}
+type Branch = Ref
+type Sha = Ref
+type Tag = Ref
+
 newFrom :: RepoLocation -> Repo
 newFrom l = 
 	Repo {
@@ -162,9 +178,9 @@ repoDescribe Repo { location = Url url } = show url
 repoDescribe Repo { location = Dir dir } = dir
 repoDescribe Repo { location = Unknown } = "UNKNOWN"
 
-{- Converts a fully qualified git ref into a user-visible version -}
-refDescribe :: String -> String
-refDescribe = remove "refs/heads/" . remove "refs/remotes/"
+{- Converts a fully qualified git ref into a user-visible version. -}
+refDescribe :: Ref -> String
+refDescribe = remove "refs/heads/" . remove "refs/remotes/" . show
 	where
 		remove prefix s
 			| prefix `isPrefixOf` s = drop (length prefix) s
@@ -432,7 +448,7 @@ useIndex index = do
 
 {- Runs an action that causes a git subcommand to emit a sha, and strips
    any trailing newline, returning the sha. -}
-getSha :: String -> IO String -> IO String
+getSha :: String -> IO String -> IO Sha
 getSha subcommand a = do
 	t <- a
 	let t' = if last t == '\n'
@@ -440,27 +456,27 @@ getSha subcommand a = do
 		else t
 	when (length t' /= shaSize) $
 		error $ "failed to read sha from git " ++ subcommand ++ " (" ++ t' ++ ")"
-	return t'
+	return $ Ref t'
 
 {- Size of a git sha. -}
 shaSize :: Int
 shaSize = 40
 
-{- Commits the index into the specified branch, 
+{- Commits the index into the specified branch (or other ref), 
  - with the specified parent refs. -}
-commit :: String -> String -> [String] -> Repo -> IO ()
+commit :: String -> Ref -> [Ref] -> Repo -> IO ()
 commit message newref parentrefs repo = do
 	tree <- getSha "write-tree" $ asString $
 		pipeRead [Param "write-tree"] repo
 	sha <- getSha "commit-tree" $ asString $
 		ignorehandle $ pipeWriteRead
-			(map Param $ ["commit-tree", tree] ++ ps)
+			(map Param $ ["commit-tree", show tree] ++ ps)
 			(L.pack message) repo
-	run "update-ref" [Param newref, Param sha] repo
+	run "update-ref" [Param $ show newref, Param $ show sha] repo
 	where
 		ignorehandle a = snd <$> a
 		asString a = L.unpack <$> a
-		ps = concatMap (\r -> ["-p", r]) parentrefs
+		ps = concatMap (\r -> ["-p", show r]) parentrefs
 
 {- Runs git config and populates a repo with its config. -}
 configRead :: Repo -> IO Repo