rev-list --boundary: fix re-injecting boundary commits.

rev-list --boundary: fix re-injecting boundary commits.

Marco reported that

	$ git rev-list --boundary --topo-order --parents 5aa44d5..ab57c8d

misses these two boundary commits.

        c649657501bada28794a30102d9c13cc28ca0e5e
        eb38cc689e84a8fd01c1856e889fe8d3b4f1bfb4

Indeed, we can see that gitk shows these two commits at the
bottom, because the --boundary code failed to output them.

The code did not check to avoid pushing the same uninteresting
commit twice to the result list.  I am not sure why this fixes
the reported problem, but this seems to fix it.

Signed-off-by: Junio C Hamano <junkio@cox.net>