bookmark: fix remote bookmark deletion when the push is raced
Before this patch, running `hg push -B book` to push the `book` bookmark
sideway at the same time as a commit making it moving forward might result in
the removal of the bookmark remotely.
After this changeset, the push can still be raced, but to remove deletion
happens. This is progress.
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).