branchcache: use update_disk to refresh 'served' and 'served.hidden'

The `update_disk` method is dedicated to this kind of usecase. Now that the writting patterns are more consistent, we can use it to warm these two important cache.

I am dropping the first comment about "refreshing all the others" because it is
false. If a branchmap already exist for "served", none of the subset will be
updated.

mercurial/localrepo.py

@@ -2923,10 +2923,9 b' class localrepository:'
 
         if repository.CACHE_BRANCHMAP_SERVED in caches:
             if tr is None or tr.changes[b'origrepolen'] < len(self):
-                # accessing the 'served' branchmap should refresh all the others,
                 self.ui.debug(b'updating the branch cache\n')
-                self.filtered(b'served').branchmap()
-                self.filtered(b'served.hidden').branchmap()
+                self._branchcaches.update_disk(self.filtered(b'served'))
+                self._branchcaches.update_disk(self.filtered(b'served.hidden'))
 
         if repository.CACHE_CHANGELOG_CACHE in caches:
             self.changelog.update_caches(transaction=tr)