add-interactive: make sure to release `rev.prune_data`

During a review, Junio Hamano pointed out that the `rev.prune_data` was
copied from another pathspec but never cleaned up.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

add-interactive.c

@@ -435,6 +435,9 @@ static int get_modified_files(struct repository *r, struct string_list *files,
 			rev.diffopt.flags.ignore_dirty_submodules = 1;
 			run_diff_files(&rev, 0);
 		}
+
+		if (ps)
+			clear_pathspec(&rev.prune_data);
 	}
 	hashmap_free_entries(&s.file_map, struct pathname_entry, ent);