bundlerepo: use _cacheabletip mechanism in bundlerepo Instead of preventing any cache write we allow writing cache for all content of the original repo. The motivation for this change is to drop the custom _writebranchcache of bundlerepo to help extraction of the branchmap logic out of localrepo.

branchmap: merge _branchtags into updatebranchcache Now that nobody overwrite it, there is no reasons for `_branchtags` to remains separated from `updatebranchcache`.

branchmap: factorise access to changelog in updatebranchcache This prepares merge of `updatebranchcache` and `_branchtags`.