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.
$ mkdir t
$ cd t
$ hg init
$ echo a > a
$ hg add a
$ hg commit -m test
$ rm .hg/requires
$ hg tip
abort: index 00changelog.i unknown format 2!
[255]
$ echo indoor-pool > .hg/requires
$ hg tip
abort: requirement 'indoor-pool' not supported!
[255]