repoview: cache filtered changelog Creating a new changelog object for each access is costly and prevents efficient caching changelog side. This introduced a x5 performance regression in log because chunk read from disk were never reused. We were jumping from about 100 disk read to about 20 000. This changeset introduce a simple cache mechanism that help the last changelog object created by a repoview. The changelog is reused until the changelog or the filtering changes. The cache invalidation is much more complicated than it should be. But MQ test show a strange cache desync. I was unable to track down the source of this desync in descent time so I'm not sure if the issue is in MQ or core. However given the proximity to the 2.5 freeze, I'm choosing the inelegant but safe route that makes the cache mechanism safer.