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.
http://example.com/no/anchor, branches: (None, [])
http://example.com/an/anchor, branches: ('foo', [])
http://example.com/no/anchor/branches, branches: (None, ['foo'])
http://example.com/an/anchor/branches, branches: ('bar', ['foo'])
http://example.com/an/anchor/branches-None, branches: ('foo', [])