Make ap_diplomat_battle() handle battle against a unit in a city

For now the only result is that the action probability for bribing a visible
unit in a city can be calculated.

See patch #4918

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@25490 a0f10bec-cc02-0410-94fc-a9cfff90b4cd