obsolete: skip 'changectx' usage in unstable computation We simplify the unstable computation code, skipping the expensive creation of changectx object. We focus on efficient set operation and revnumber centric functions. In my mercurial development repository, this provides a 3x speedup to the function: before: 5.319 ms after: 1.844 ms repo details: total changesets: 40886 obsolete changesets: 7756 mutable (not obsolete): 293 unstable: 30