improve back-compat of post-receive hook

* init: Improve generated post-receive hook, so it won't fail when
  run on a system whose git-annex is too old to support git-annex post-receive
* init: Update the post-receive hook when re-run in an existing repository.

This commit was sponsored by Jack Hill on Patreon.

Git/Hook.hs

@@ -1,6 +1,6 @@
 {- git hooks
  -
- - Copyright 2013-2015 Joey Hess <id@joeyh.name>
+ - Copyright 2013-2018 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU GPL version 3 or higher.
  -}
@@ -20,6 +20,7 @@ import Utility.FileMode
 data Hook = Hook
 	{ hookName :: FilePath
 	, hookScript :: String
+	, hookOldScripts :: [String]
 	}
 	deriving (Ord)
 
@@ -30,38 +31,47 @@ hookFile :: Hook -> Repo -> FilePath
 hookFile h r = localGitDir r </> "hooks" </> hookName h
 
 {- Writes a hook. Returns False if the hook already exists with a different
- - content. -}
+ - content. Upgrades old scripts. -}
 hookWrite :: Hook -> Repo -> IO Bool
-hookWrite h r = do
-	let f = hookFile h r
-	ifM (doesFileExist f)
-		( expectedContent h r
-		, do
-			viaTmp writeFile f (hookScript h)
-			p <- getPermissions f
-			setPermissions f $ p {executable = True}
-			return True
-		)
+hookWrite h r = ifM (doesFileExist f)
+	( expectedContent h r >>= \case
+		UnexpectedContent -> return False
+		ExpectedContent -> return True
+		OldExpectedContent -> go
+	, go
+	)
+  where
+	f = hookFile h r
+	go = do
+		viaTmp writeFile f (hookScript h)
+		p <- getPermissions f
+		setPermissions f $ p {executable = True}
+		return True
 
 {- Removes a hook. Returns False if the hook contained something else, and
  - could not be removed. -}
 hookUnWrite :: Hook -> Repo -> IO Bool
-hookUnWrite h r = do
-	let f = hookFile h r
-	ifM (doesFileExist f)
-		( ifM (expectedContent h r)
-			( do
-				removeFile f
-				return True
-			, return False
-			)
-		, return True
-		)
+hookUnWrite h r = ifM (doesFileExist f)
+	( expectedContent h r >>= \case
+		UnexpectedContent -> return False
+		_ -> do
+			removeFile f
+			return True
+	, return True
+	)
+  where
+	f = hookFile h r
 
-expectedContent :: Hook -> Repo -> IO Bool
+data ExpectedContent = UnexpectedContent | ExpectedContent | OldExpectedContent
+
+expectedContent :: Hook -> Repo -> IO ExpectedContent
 expectedContent h r = do
 	content <- readFile $ hookFile h r
-	return $ content == hookScript h
+	return $ if content == hookScript h
+		then ExpectedContent
+		else if any (content ==) (hookOldScripts h)
+			then OldExpectedContent
+			else UnexpectedContent
 
 hookExists :: Hook -> Repo -> IO Bool
 hookExists h r = do