better key to file mapping

Locations.hs

@@ -20,10 +20,17 @@ gitStateDir :: GitRepo -> FilePath
 gitStateDir repo = (gitWorkTree repo) ++ "/" ++ stateLoc ++ "/"
 
 {- Converts a key into a filename fragment.
- - Just escape "/" in the key name, to keep a flat
+ -
- - tree of files and avoid issues with files ending with "/" etc. -}
+ - Escape "/" in the key name, to keep a flat tree of files and avoid
+ - issues with keys containing "/../" or ending with "/" etc. 
+ -
+ - "/" is escaped to "%" because it's short and rarely used, and resembles
+ -     a slash
+ - "%" is escaped to "&s", and "&" to "&a"; this ensures that the mapping
+ -     is one to one.
+ - -}
 keyFile :: Key -> FilePath
-keyFile key = replace "/" "&s" $ replace "&" "&a" key
+keyFile key = replace "/" "%" $ replace "%" "%s" $ replace "&" "&a" key
 
 {- An annexed file's content is stored in 
  - .git/annex/<backend>/<key> ; this allows deriving the key and backend
@@ -32,6 +39,7 @@ annexLocation :: State -> Backend -> Key -> FilePath
 annexLocation state backend key = 
 	(gitWorkTree $ repo state) ++ "/" ++ 
 	(annexLocationRelative state backend key)
+
 annexLocationRelative :: State -> Backend -> Key -> FilePath
 annexLocationRelative state backend key =
 	gitDir (repo state) ++ "/annex/" ++ (name backend) ++