downgrade: if a compression is removed, consider that too For compression format variant, the result of `fromrepo()` and `fromconfig()` is not a boolean and we have to actually equate those to find change. Differential Revision: https://phab.mercurial-scm.org/D9693

engine: prevent a function call for each store file Python function calls are not cheap. Instead of calling the function once for each file in store, we use a dedicated function which filters and yields the required files. Differential Revision: https://phab.mercurial-scm.org/D9673