localrepo: add isfilecached to check filecache-ed property is already cached isfilecached() encapsulates internal implementation of filecache-ed property. "name in repo.unfiltered().__dict__" or so can't be used for this purpose, because corresponded entry in __dict__ might be discarded by repo.invalidate(), repo.invalidatedirstate() or so (fsmonitor does so, for example). This patch makes isfilecached() return not only whether filecache-ed property is already cached, but also already cached value (or None), in order to avoid subsequent access to cached object via "repo.NAME", which prevents main Mercurial procedure after reposetup() from validating cache.