git-rev-list --merge-order hangs

I've got a Linux git tree with some merges and some other commits
interwined.
I want to print all my changes in merge order, so I do:

git-rev-list --pretty --merge-order v2.6.12-rc6-astorm1 ^v2.6.12-rc6

It prints the first 3 commits (linear), but then hangs, eating all
available CPU.
Without --merge-order it works fine, but obviously lists the commits by
parent.

AstralStorm