bookmark: introduce in advance a variant of the exchange test We are about to introduce a new way to push bookmark to server. We introduce the test variant before actually updating the exchange to help the output changes to stand out when it happens.

push: move bundle2-pushkey based bookmarks exchange in its own function We are about to introduce an alternative way to push bookmark over bundle2.

bookmark: add pushkey hook compatiblity to the bundle2 part Currently, pushing a bookmark update triggers a pushkey hooks. It is likely that users in the wild use such hooks to control bookmark movement. Using a non push-key mechanism to exchange bookmark means these hooks are no longer called, possibly breaking existing users setup. So we add explicit call to the pushkey hooks in the handling of the bundle2 part. This behavior can be disabled with a new config knob: 'server.bookmarks-pushkey-compat'.

bookmark: introduce a 'bookmarks' part This part can carry and apply bookmarks information. We start with adding the core behavior of the part. In its current form, the part is only suitable for push since it plain update the bookmark without consideration for the local state. Support of the behavior needed for pulling will be added in later changesets.

push: include a 'check:bookmarks' part when possible Before updating the actual bookmark update, we can start with updating the way we check for push race. Checking bookmarks state earlier is useful even if we still use pushkey. Aborting before the changegroup is added can save a lot of time.

bookmark: add a 'check:bookmarks' bundle2 part This part checks that bookmarks are still at the node they are expected to be. This allows a pushing client to detect push race where the repository was updated between the time it discovered the server state and the time it managed to finish its push. Such checking already exists when pushing bookmark through pushkey. This new part can be inserted at the beginning of the bundle, triggering abort earlier. In addition, we would like to move away from pushey to push bookmark. A step useful to solve issue5165.

bookmark: add methods to binary encode and decode bookmark values Coming new bundle2 parts related to bookmark will use a binary encoding. It encodes a series of '(bookmark, node)' pairs. Bookmark name has a high enough size limit to not be affected by issue5165. (64K length, we are well covered)