tests/test-empty-dir.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.

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0 files updated, 0 files merged, 1 files removed, 0 files unresolved
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