help: fix example of revs() fileset

filecache: make 'join' abstract All subclasses redefine this method, so we can make it abstract.

filecache: explicitly test 'repofilecache' The tests is actually about testing a repofilecache (it uses a fake repo). We make this clear to prevent blockers while cleaning theses API.

repofilecache: directly use 'repo.vfs.join' The 'vfs' attribute already have all methods we need, the value of going through the repository for this is low. so we removes it.

repofilecache: define a 'join' method We are about to turn the 'join' method of the base class Abstract, so we need on to be defined in the localrepo. The ultimate goal here is to be able to stop relying for the 'localrepo' class to have a 'join' methods (there is above one hundred methods on 'localrepo'. This change make te 'repo' file cache have its own code so that we can prepare this change to the repostory class. explicite join

wix: add censor docs to installer script Spotted by Matt Harbison.