Always check for directory changes with sendfile auto completion

Instead of only checking for files when 'last_directory' has changed, do
it every time.

Add autocomplete_update function that updates the items while retaining
last_found and search_str.

Fixes #1099

src/tools/autocomplete.h

@@ -51,6 +51,7 @@ void autocomplete_free(Autocomplete ac);
 
 void autocomplete_add(Autocomplete ac, const char *item);
 void autocomplete_add_all(Autocomplete ac, char **items);
+void autocomplete_update(Autocomplete ac, char **items);
 void autocomplete_remove(Autocomplete ac, const char *const item);
 void autocomplete_remove_all(Autocomplete ac, char **items);