branchmap: use set for update code We are doing membership test and substraction. new code is marginally faster.

branchmap: simplify update code We drop iterrevs which are not needed anymore. The know head are never a descendant of the updated set. It was possible with the old strip code. This simplification make the code easier to read an update.

branchmap: stop useless rev -> node -> rev round trip We never use the node of new revisions unless in the very specific case of closed heads. So we can just use the revision number. So give another handfull of percent speedup.