bundlerepo: don't write branch cache to disk

Calling branchmap() or similar on a bundlerepo would write the bundle-augmented
branch cache to disk, requiring a subsequent expensive rebuild when the repo
is used without the bundle.

mercurial/bundlerepo.py

@@ -266,6 +266,10 b' class bundlerepository(localrepo.localre'
     def getcwd(self):
         return os.getcwd() # always outside the repo
 
+    def _writebranchcache(self, branches, tip, tiprev):
+        # don't overwrite the disk cache with bundle-augmented data
+        pass
+
 def instance(ui, path, create):
     if create:
         raise util.Abort(_('cannot create new bundle repository'))