tests/test-revert-unknown.out

bookmarks: Avoid unconditional forwarding of bookmarks for the null revision

This patch fixes the following minor problem:

% hg bookmark -r null test
% hg bookmarks
   test                      -1:000000000000
[...]
% hg commit
% hg bookmarks
 * test                      17:861ce7a241f9

That is: Bookmarks referring to the null revision are always forwarded
to the current revision.

%% Should show unknown
? unknown
removing b
%% Should show unknown and b removed
R b
? unknown
%% Should show a and unknown
a
unknown