implement massReplace

This looks at the string one char at a time, which is hardly efficient..
but more than good enough for expanding variables in
relatively short command lines.

Annex/Content.hs

@@ -447,12 +447,12 @@ downloadUrl urls file = go =<< annexWebDownloadCommand <$> Annex.getGitConfig
 		liftIO $ anyM (\u -> Url.download u headers opts file) urls
 	go (Just basecmd) = liftIO $ anyM (downloadcmd basecmd) urls
 	downloadcmd basecmd url =
-		boolSystem "sh" [Param "-c", Param $ gencmd basecmd url]
+		boolSystem "sh" [Param "-c", Param $ gencmd url basecmd]
 			<&&> doesFileExist file
-	gencmd basecmd url = 
-		replace "%file" (shellEscape file) $
-			replace "%url" (shellEscape url)
-				basecmd
+	gencmd url = massReplace
+		[ ("%file", shellEscape file)
+		, ("%url", shellEscape url)
+		]
 
 {- Copies a key's content, when present, to a temp file.
  - This is used to speed up some rsyncs. -}

Utility/Misc.hs

@@ -11,6 +11,7 @@ import System.IO
 import Control.Monad
 import Foreign
 import Data.Char
+import Data.List
 import Control.Applicative
 import System.Posix.Process (getAnyProcessStatus)
 
@@ -70,6 +71,23 @@ segmentDelim p l = map reverse $ go [] [] l
 		| p i = go [] ([i]:c:r) is
 		| otherwise = go (i:c) r is
 
+{- Replaces multiple values in a string.
+ -
+ - Takes care to skip over just-replaced values, so that they are not
+ - mangled. For example, massReplace [("foo", "new foo")] does not
+ - replace the "new foo" with "new new foo".
+ -}
+massReplace :: [(String, String)] -> String -> String
+massReplace vs = go [] vs
+  where
+
+	go acc _ [] = concat $ reverse acc
+	go acc [] (c:cs) = go ([c]:acc) vs cs
+	go acc ((val, replacement):rest) s
+		| val `isPrefixOf` s =
+			go (replacement:acc) vs (drop (length val) s)
+		| otherwise = go acc rest s
+
 {- Given two orderings, returns the second if the first is EQ and returns
  - the first otherwise.
  -