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.