getchangegroup: take an 'outgoing' object as argument (API) There is various version of this function that differ mostly by the way they define the bundled set. The flexibility is now available in the outgoing object itself so we move the complexity into the caller themself. This will allow use to remove a good share of the similar function to obtains a changegroup in the 'changegroup.py' module. An important side effect is that we stop calling 'computeoutgoing' in 'getchangegroup'. This is fine as code that needs such argument processing is actually going through the 'exchange' module which already all this function itself.

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.

outgoing: adds some default value for argument We are about to introduce a third option to create an outgoing object: 'missingroots'. This argument will be mutually exclusive with 'commonheads' so we implement some default value handling in preparation. This will also help use to make more use of outgoing creation around the code base.