plug a few coverity-spotted leaks

Signed-off-by: Jim Meyering <meyering@redhat.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/diff.c

@@ -182,6 +182,7 @@ static int builtin_diff_combined(struct rev_info *revs,
 		hashcpy((unsigned char *)(parent + i), ent[i].item->sha1);
 	diff_tree_combined(parent[0], parent + 1, ents - 1,
 			   revs->dense_combined_merges, revs);
+	free(parent);
 	return 0;
 }