pull: perform bookmark updates in the transaction

bookmark: add a `bmstore.recordupdate` to plug bookmarks into the transaction Instead of manually writing bookmarks when they are updated, we can just record this update to the transaction and rely on it to update the on-disk file.

bookmarks: split bookmark serialization and file handling If we want to handle bookmarks in a transaction we need to decouple the file handling and the actual production of the content. This is similar to how we handle phases in transaction.

transaction: allow generating file outside of store We allow a vfs argument to `addfilegenerator`. This allows generating files outside of the store directory like bookmarks. However, this is not really working since we do not have the infrastructure to backup and restore files outside of store. By chance, the bookmark file is already backed up by another mechanism so we can restrict this new feature to bookmarks (which is our only interest here) and proceed.

transaction: work around and document issue with file backup The backup restoration is actually hard-coded for the main file. And this hard-coded list is the only one used when repairing an interrupted transaction from another process. Solving this problem is out of the scope of this series so we document it and work around its implications.

bundle2: remove an explicit packing The final writing of the empty part was done explicitly. We now using proper pack call using symbolic constant. This open simple change in the bundle2 format.