diff.c: do not pass diff options as keydata to hashmap

When we initialize the hashmap, we give it a pointer to the
diff_options, which it then passes along to each call of the
hashmap_cmp_fn function. There's no need to pass it a second time as
the "keydata" parameter, and our comparison functions never look at
keydata.

This was a mistake left over from an earlier round of 2e2d5ac1
(diff.c: color moved lines differently, 2017-06-30), before hashmap
learned to pass the data pointer for us.
Explanation-by: NJeff King <peff@peff.net>
Signed-off-by: NStefan Beller <sbeller@google.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

diff.c

@@ -842,13 +842,13 @@ static void mark_color_as_moved(struct diff_options *o,
 		case DIFF_SYMBOL_PLUS:
 			hm = del_lines;
 			key = prepare_entry(o, n);
-			match = hashmap_get(hm, key, o);
+			match = hashmap_get(hm, key, NULL);
 			free(key);
 			break;
 		case DIFF_SYMBOL_MINUS:
 			hm = add_lines;
 			key = prepare_entry(o, n);
-			match = hashmap_get(hm, key, o);
+			match = hashmap_get(hm, key, NULL);
 			free(key);
 			break;
 		default: