localrepo: always write the filtered phasecache when nodes are destroyed (issue3827) When the strip command is run, it calls repo.destroyed, which in turn checks if we read _phasecache, and if we did calls filterunknown on it and flushes the changes immediately. But in some cases, nothing causes _phasecache to be read, so we miss out on this and the file remains the same on-disk. Then a call to invalidate comes, which should refresh _phasecache if it changed, but it didn't, so it keeps using the old one with the stripped revision which causes an IndexError. Test written by Yuya Nishihara.