Update player kills the connection to unit in game

[steveoc64]

Each time a user record is updated by admin, this stamps the game_unit_stats records with the owning player.

That is a good thing, however, it does not take into account sub-commanders.

If you alter a top level user account (to set a new theme or whatever), then the sub-commanders lose their connection to the units.

Change this so that it only does the unit-player stamping IF and only if the commander_id changes. Otherwise, dont update anything in the game_unit_stats table.