bookmarks: issue a warning if remote doesn't support comparing bookmarks
We want to issue a warning and abort comparing bookmarks if remote doesn't
support it. Otherwise hg out -B will list you outgoing bookmarks that cannot
be pushed to the remote repository using hg push -B.
Ancestors of 5
4 2 0
Ancestors of 6 and 5
3 4 2 1 0
Ancestors of 5 and 4
4 2 0
Descendants of 5
7 8
Descendants of 5 and 3
6 7 8
Descendants of 5 and 4
5 7 8