branchmap: move branch cache code out of streamclone.py This is low-level branch map and cache manipulation code. It deserves to live next to similar code in branchmap.py. Moving it also paves the road for multiple consumers, such as a bundle2 part handler that receives branch mappings from a remote. This is largely a mechanical move, with only variable names and indentation being changed.

streamclone: move streamin() into maybeperformstreamclone() streamin() only had a single consumer. And it always only ever will because it is strongly coupled with the current, soon-to-be-superseded-by-bundle2 functionality. The return value has been dropped because nobody was using it.

streamclone: refactor maybeperformstreamclone to take a pullop Just like all the other pull steps. Consistency is good. This seems a little excessive right now since maybeperformstreamclone is such a short function. This will be addressed in a subsequent patch.