Revert "remove absNormPathUnix, using my absPathFrom replacement"

This reverts commit a7f05c007b.

Consider: relPathDirToFile (absPathFrom "/tmp/repo/xxx" "y/bar") "/tmp/repo/.git/annex/objects/xxx"

This needs to always yield "../../../.git/annex/objects/xxx" but on
Windows, it is "..\\..\\/tmp/repo/.git/annex/objects/xxx"

Locations.hs

@@ -144,9 +144,11 @@ gitAnnexLocation' key r crippled
 gitAnnexLink :: FilePath -> Key -> Git.Repo -> IO FilePath
 gitAnnexLink file key r = do
 	currdir <- getCurrentDirectory
-	let absfile = absPathFrom currdir file
+	let absfile = fromMaybe whoops $ absNormPathUnix currdir file
 	loc <- gitAnnexLocation' key r False
 	relPathDirToFile (parentDir absfile) loc
+  where
+	whoops = error $ "unable to normalize " ++ file
 
 {- File used to lock a key's content. -}
 gitAnnexContentLock :: Key -> Git.Repo -> GitConfig -> IO FilePath

Utility/Path.hs

@@ -5,7 +5,7 @@
  - License: BSD-2-clause
  -}
 
-{-# LANGUAGE CPP #-}
+{-# LANGUAGE PackageImports, CPP #-}
 
 module Utility.Path where
 
@@ -24,6 +24,7 @@ import System.Posix.Files
 import Utility.Exception
 #endif
 
+import qualified "MissingH" System.Path as MissingH
 import Utility.Monad
 import Utility.UserInfo
 
@@ -64,6 +65,18 @@ simplifyPath path = dropTrailingPathSeparator $
 absPathFrom :: FilePath -> FilePath -> FilePath
 absPathFrom dir path = simplifyPath (combine dir path)
 
+{- On Windows, this converts the paths to unix-style, in order to run
+ - MissingH's absNormPath on them. Resulting path will use / separators. -}
+absNormPathUnix :: FilePath -> FilePath -> Maybe FilePath
+#ifndef mingw32_HOST_OS
+absNormPathUnix dir path = MissingH.absNormPath dir path
+#else
+absNormPathUnix dir path = todos <$> MissingH.absNormPath (fromdos dir) (fromdos path)
+  where
+	fromdos = replace "\\" "/"
+	todos = replace "/" "\\"
+#endif
+
 {- takeDirectory "foo/bar/" is "foo/bar". This instead yields "foo" -}
 parentDir :: FilePath -> FilePath
 parentDir = takeDirectory . dropTrailingPathSeparator