avoid using openFile when withFile can be used

Potentially fixes some FD leak if an action on an opened file handle fails
for some reason. There have been some hard to reproduce reports of
git-annex leaking FDs, and this may solve them.

Annex/Link.hs

@@ -51,19 +51,15 @@ getAnnexLinkTarget file = ifM (coreSymlinks <$> Annex.getGitConfig)
 				| otherwise -> return Nothing
 			Nothing -> fallback
 
-	probefilecontent f = do
-		h <- openFile f ReadMode
+	probefilecontent f = withFile f ReadMode $ \h -> do
 		fileEncoding h
 		-- The first 8k is more than enough to read; link
 		-- files are small.
 		s <- take 8192 <$> hGetContents h
 		-- If we got the full 8k, the file is too large
 		if length s == 8192
-			then do
-				hClose h
-				return ""
-			else do
-				hClose h
+			then return ""
+			else 
 				-- If there are any NUL or newline
 				-- characters, or whitespace, we
 				-- certianly don't have a link to a