branchmap: drop `_cacheabletip` usage in `updatecache` Nobody overwrite the `_cacheabletip` any more. We always update the cache for the whole repo and write it to disk (or at list try to). The `updatecache` code is simplied to remove the double phase logic associated with _cacheabletip.

basic:

creating
creating
creating
creating

fakeuncacheable:

creating
creating
creating
creating
creating
creating