bundle2: add a 'modes' dictionary to the bundle operation This new attribute allows the codes requesting an unbundling to pass important information to individual part handlers. The current target use case is to allow for receiving 'bookmarks' part without directly updating local repository, but just recording the received data instead. This is necessary for pull where the remote bookmarks are processed locally. I expect the concept to be beneficial to other parts in the future. To clarify the bookmark behavior on pull, the remote bookmark value are not just taken -as-is- into the local repository. There is an extra step to detect bookmark divergence. The remote bookmarks data are stored until this processing happens.

bookmark: use the 'bookmarks' bundle2 part to push bookmark update (issue5165) We use the new binary parts we introduced earlier to exchange bookmark. The payload is a bit more compact since we use binary and the length of bookmarks is no longer constrained to 255. .. fix:: Issue 5165 Bookmark, whose name is longer than 255, can again be exchanged again between 4.4+ client and servers.

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.