diff-files: fix incorrect usage of an empty tree

In c26022e (diff: convert diff_addremove to struct object_id,
2017-05-30), the OID to use for intent-to-add files was inadvertently
changed from the empty blob to the empty tree. Let's revert that.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

diff-lib.c

@@ -220,7 +220,7 @@ int run_diff_files(struct rev_info *revs, unsigned int option)
 			} else if (revs->diffopt.ita_invisible_in_index &&
 				   ce_intent_to_add(ce)) {
 				diff_addremove(&revs->diffopt, '+', ce->ce_mode,
-					       the_hash_algo->empty_tree, 0,
+					       the_hash_algo->empty_blob, 0,
 					       ce->name, 0);
 				continue;
 			}