pull: gather explicit bookmark pulls with bookmark updates There is no reason to make them at different times. So we gather them. This is the first step toward merging them.

subrepo: stop pulling bookmark manually Bookmark pulling is now done with all the other pull steps in the `exchange.pull` function.

pull: retrieve bookmarks through bundle2 We can retrieve any pushkey namespace through bundle2. So we also ask for bookmark data and use them to update the local repo.

pull: retrieve bookmarks before obsmarkers Retrieving bookmarks before obsmarkers will avoid turning some changesets hidden right before making them visible again if a bookmark keeps them visible.