git-rev-list problem with --max-age and --merge-order

I have a version of gitk that draws stuff more-or-less as it receives
it from git-rev-list -- it reorders the commits somewhat but it
doesn't need to wait for the end of the output from git-rev-list
before it can draw anything.  To do that it passes --merge-order to
git-rev-list and relies on the property that parents will be listed
after all of their children.

I also want to make the default be to show the commits for the last 30
days.  However, using --max-age with --merge-order outputs many fewer
commits than I get if I use the same --max-age argument without
--merge-order.  For example, on the current linux-2.6 git tree,
git-rev-tree --max-age=1116330140 HEAD outputs 655 commits, whereas
git-rev-tree --merge-order --max-age=1116330140 HEAD only outputs 173
commits.  That date is 2005-05-17 21:42:20 in my timezone, about 30
days ago.  An example of a commit that isn't output with --merge-order
is 1e86d1c648508fd50e6c9960576b87906a7906ad:

Author: Benjamin Herrenschmidt [off-list ref]  2005-06-02 14:11:37
Committer: Linus Torvalds [off-list ref]  2005-06-03 01:19:27

[PATCH] ppc64: Fix result code handling in prom_init

Any ideas why using --merge-order means we don't see this one?  It's
certainly less than 30 days old.

Thanks,
Paul.