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.
Test issue2761
$ hg init repo
$ cd repo
$ touch to-be-deleted
$ hg add
adding to-be-deleted
$ hg ci -m first
$ echo a > to-be-deleted
$ hg ci -m second
$ rm to-be-deleted
$ hg diff -r 0
Same issue, different code path
$ hg up -C
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ touch does-not-exist-in-1
$ hg add
adding does-not-exist-in-1
$ hg ci -m third
$ rm does-not-exist-in-1
$ hg diff -r 1