fix git 1.7.7 breakage

* This version of git-annex only works with git 1.7.7 and newer.
  The breakage with old versions is subtle, and affects
  annex.numcopies .gitattributes settings, so be sure to upgrade git
  to 1.7.7. (Debian package now depends on that version.)
* Don't pass absolute paths to git show-attr, as it started following
  symlinks when that's done in 1.7.7. Instead, use relative paths,
  which show-attr only handles 100% correctly in 1.7.7. Closes: #645046

Unfortunatly I can find no way to work with the old and new gits, as
the old had bugs that require absolute paths, while the new doesn't like
them at all. And the behavior of git show-attr in 1.7.7. is the same as
eg, git add of an absolute path to a symlink, so seems entirely
intentional and not likely to change.

Git.hs

@@ -547,36 +547,23 @@ configMap = config
 {- Efficiently looks up a gitattributes value for each file in a list. -}
 checkAttr :: Repo -> String -> [FilePath] -> IO [(FilePath, String)]
 checkAttr repo attr files = do
-	-- git check-attr wants files that are absolute (or relative to the
-	-- top of the repo). But we're passed files relative to the current
-	-- directory. Convert to absolute, and then convert the filenames
-	-- in its output back to relative.
-	cwd <- getCurrentDirectory
-	let top = workTree repo
-	let absfiles = map (absPathFrom cwd) files
 	(_, fromh, toh) <- hPipeBoth "git" (toCommand params)
         _ <- forkProcess $ do
 		hClose fromh
-                hPutStr toh $ join "\0" absfiles
+                hPutStr toh $ join "\0" files
                 hClose toh
                 exitSuccess
         hClose toh
-	s <- hGetContents fromh
-	return $ map (topair cwd top) $ lines s
+	(map topair . lines) <$> hGetContents fromh
 	where
 		params = gitCommandLine repo [Param "check-attr", Param attr, Params "-z --stdin"]
-		topair cwd top l = (relfile, value)
+		topair l = (file, value)
 			where 
-				relfile
-					| startswith cwd' file = drop (length cwd') file
-					| otherwise = relPathDirToFile top' file
 				file = decodeGitFile $ join sep $ take end bits
 				value = bits !! end
 				end = length bits - 1
 				bits = split sep l
 				sep = ": " ++ attr ++ ": "
-				cwd' = cwd ++ "/"
-				top' = top ++ "/"
 
 {- Some git commands output encoded filenames. Decode that (annoyingly
  - complex) encoding. -}