saves: Add a po comment to the string "replay".

src/save_index.cpp

@@ -233,6 +233,9 @@ std::string save_info::format_time_summary() const
 
 bool save_info_less_time::operator()(const save_info& a, const save_info& b) const
 {
+	// This translatable string must be same one as in replay_savegame::create_initial_filename.
+	// TODO: we really shouldn't be relying on translatable strings like this, especially since
+	// old savefiles may have been created in a different language than the current UI language
 	const std::string replay_str = " " + _("replay");
 	if(a.modified() > b.modified()) {
 		return true;

src/savegame.cpp

@@ -544,6 +544,7 @@ replay_savegame::replay_savegame(saved_game &gamestate, const compression::forma
 
 std::string replay_savegame::create_initial_filename(unsigned int) const
 {
+	// TRANSLATORS: This string is used as part of a filename, as in, "HttT-The Elves Besieged replay.gz"
 	return formatter() << gamestate().classification().label << " " << _("replay");
 }