histedit: move topmost bookmark movement to a separate function histedit treats topmost bookmark movement specially. The rest of the bookmark movement could be handled by scmutil.cleanupnodes. So let's move the special logic out to make the patch easier to review.

histedit: remove moving bookmarks message on verbose (BC) This is more consistent with other commands, like "commit -v" won't show bookmark movement messages. It will make migrating to scmutil.cleanupnodes easier.