share: share 'cachevfs' with the source clone (issue5108) Share extension now also share caches reads and writes. Not sharing caches results in costly caches recomputations which can takes up to minutes when using shares on large repositories. There are a couple of file in the '.hg/cache/' that depends of the current visibility. Visibility can be affected by the working copy location, something which is specific to each share. We ignores them for this series because they: * are the minority, * already have a good fallback to other precomputed caches, * are only affected when people use the experimental evolution feature.