Groundwar pathfinding: moving target

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In ground war, when multiple units receive the same target, the pathfinding 
ignores the group and plans a path accordingly. 

However, when their target moves away, each unit detects this individually and 
performs independent pathfinding to the new location, but this time, they 
consider each other as non-passable cells.

One fix would be that if one unit detects its target to move, select all units 
with the same target and do a group pathfinding similar to the one with the 
initial user command.

Original issue reported on code.google.com by [email protected] on 21 May 2013 at 5:13