Prevent key names from containing newlines.

There are several places where it's assumed a key can be written on one
line. One is in the format of the .git/annex/unused files. The difficult
one is that filenames derived from keys are fed into git cat-file --batch,
which has a line based input. (And no -z option.)

So, for now it's best to block such keys being created.

Backend.hs

@@ -64,7 +64,13 @@ genKey' (b:bs) file = do
 	r <- (B.getKey b) file
 	case r of
 		Nothing -> genKey' bs file
-		Just k -> return $ Just (k, b)
+		Just k -> return $ Just (makesane k, b)
+	where
+		-- keyNames should not contain newline characters.
+		makesane k = k { keyName = map fixbadchar (keyName k) }
+		fixbadchar c
+			| c == '\n' = '_'
+			| otherwise = c
 
 {- Looks up the key and backend corresponding to an annexed file,
  - by examining what the file symlinks to. -}