split more stuff out of Git.hs

Git/Sha.hs

@@ -0,0 +1,27 @@
+{- git SHA stuff
+ -
+ - Copyright 2011 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Git.Sha where
+
+import Common
+import Git.Types
+
+{- 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 Sha
+getSha subcommand a = do
+	t <- a
+	let t' = if last t == '\n'
+		then init t
+		else t
+	when (length t' /= shaSize) $
+		error $ "failed to read sha from git " ++ subcommand ++ " (" ++ t' ++ ")"
+	return $ Ref t'
+
+{- Size of a git sha. -}
+shaSize :: Int
+shaSize = 40