outgoing: add a 'missingroots' argument...
outgoing: add a 'missingroots' argument
This argument can be used instead of 'commonheads' to determine the 'outgoing'
set. We remove the outgoingbetween function as its role can now be handled by
'outgoing' itself.
I've thought of using an external function instead of making the constructor
more complicated. However, there is low hanging fruit to improve the current
code flow by storing some side products of the processing of 'missingroots'. So
in my opinion it make senses to add all this to the class.
