Help, how did Mozilla do this?

[Johnny P]

Ref: http://hg.mozilla.org/integration/gaia/graph

In the picture below I found Mozilla's mercurial repository graph with a
situation that I thought was not possible. They have two commits that have
the same parent where one is a merge. I have wanted to do this, but cannot
find an answer on how to, can anyone help?

    @ merge pull request from foo polish text
    |\
    | o bug 1 - add text ...
    |/
    o merge pull request from bar bug 2

How did they get this to occur? Usually when you pull changes in that add
onto the head, if you attempt to merge you get "nothing to merge".

Thanks
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