branchcache: improve speed relative to the amount of heads Updating the branch cache is quadratic to the amount of heads in the repository. One consequence of this was that cloning a pathological repository with 10,000 heads (and nothing else) took hours of CPU time. This patch makes one of the inner loop much faster, by removing a changectx instantiation, and removes another entirely in cases where there are no candidate branch heads which descend from other branch heads.