merge-tree: fix typo in merge-tree.c::unresolved

When calculating whether there is a d/f conflict, the calculation of
whether both sides are directories generates an incorrect references
mask because it does not use the loop index to set the correct bit.
Fix this typo.

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/merge-tree.c

@@ -245,7 +245,7 @@ static void unresolved(const struct traverse_info *info, struct name_entry n[3])
 	unsigned dirmask = 0, mask = 0;
 
 	for (i = 0; i < 3; i++) {
-		mask |= (1 << 1);
+		mask |= (1 << i);
 		if (n[i].mode && S_ISDIR(n[i].mode))
 			dirmask |= (1 << i);
 	}