repoview: split _gethiddenblockers Split up _gethiddenblockers into two categories: (1) "static' blockers that solely rely on the contents of obstore and are visible children of hidden changsets. (2) "dynamic" blockers, appearing by having wd parents, bookmarks or tags pointing to hidden changesets. We assume that (1) doesn't change often and can be easily cached with a good invalidation strategy. (2) change often, but barely produce blockers, so we can recompute them if necessary.