diffdriver: Added --text option for easy diffing of the contents of annexed text files

This was already possible, but it was rather hard to come up with the
complex shell command needed.

Note that the diff output starts with "diff a/... b/...".
I left off the "--git" because it's not a git format diff.

Command/DiffDriver.hs

@@ -1,6 +1,6 @@
 {- git-annex command
  -
- - Copyright 2014-2021 Joey Hess <id@joeyh.name>
+ - Copyright 2014-2023 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU AGPL version 3 or higher.
  -}
@@ -15,13 +15,26 @@ import Git.Types
 cmd :: Command
 cmd = dontCheck repoExists $
 	command "diffdriver" SectionPlumbing 
-		"external git diff driver shim"
-		("-- cmd --") (withParams seek)
+		"git diff driver"
+		("-- cmd --") (seek <$$> optParser)
 
-seek :: CmdParams -> CommandSeek
-seek = withWords (commandAction . start)
+data Options = Options
+	{ textDiff :: Bool
+	, restOptions :: CmdParams
+	}
 
-start :: [String] -> CommandStart
+optParser :: CmdParamsDesc -> Parser Options
+optParser desc = Options
+	<$> switch
+		( long "text"
+		<> help "diff text files with diff(1)"
+		)
+	<*> cmdParams desc
+
+seek :: Options -> CommandSeek
+seek = commandAction . start
+
+start :: Options -> CommandStart
 start opts = do
 	let (req, differ) = parseReq opts
 	void $ liftIO . exitBool =<< liftIO . differ =<< fixupReq req
@@ -55,10 +68,12 @@ serializeReq req@(Req {}) = map Param
 	, rNewMode req
 	]
 
-parseReq :: [String] -> (Req, Differ)
-parseReq opts = case separate (== "--") opts of
-	(c:ps, l) -> (mk l, externalDiffer c ps)
-	([],_) -> badopts
+parseReq :: Options -> (Req, Differ)
+parseReq opts
+	| textDiff opts = (mk (restOptions opts), textDiffer)
+	| otherwise = case separate (== "--") (restOptions opts) of
+		(c:ps, l) -> (mk l, externalDiffer c ps)
+		([],_) -> badopts
   where
 	mk (path:old_file:old_hex:old_mode:new_file:new_hex:new_mode:[]) =
 		Req
@@ -73,7 +88,7 @@ parseReq opts = case separate (== "--") opts of
 	mk (unmergedpath:[]) = UnmergedReq { rPath = unmergedpath }
 	mk _ = badopts
 
-	badopts = giveup $ "Unexpected input: " ++ unwords opts
+	badopts = giveup $ "Unexpected input: " ++ unwords (restOptions opts)
 
 {- Check if either file is a symlink to a git-annex object,
  - which git-diff will leave as a normal file containing the link text.
@@ -101,3 +116,14 @@ fixupReq req@(Req {}) =
 
 externalDiffer :: String -> [String] -> Differ
 externalDiffer c ps = \req -> boolSystem c (map Param ps ++ serializeReq req )
+
+textDiffer :: Differ
+textDiffer req = do
+	putStrLn ("diff a/" ++ rPath req ++ " b/" ++ rPath req)
+	-- diff exits nonzero on difference, so ignore exit status
+	void $ boolSystem "diff"
+		[ Param "-u"
+		, Param (rOldFile req)
+		, Param (rNewFile req)
+		]
+	return True