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/SHA.hs

@@ -90,10 +90,12 @@ keyValueE size file = keyValue size file >>= maybe (return Nothing) addE
 			, keyBackendName = shaNameE size
 			}
 		naiveextension = takeExtension file
-		extension = 
-			if length naiveextension > 6
-				then "" -- probably not really an extension
-				else naiveextension
+		extension
+			-- long or newline containing extensions are 
+			-- probably not really an extension
+			| length naiveextension > 6 ||
+			  '\n' `elem` naiveextension = ""
+			| otherwise = naiveextension
 
 {- A key's checksum is checked during fsck. -}
 checkKeyChecksum :: SHASize -> Key -> Annex Bool