You can use the git merge tool. I'm sorry but we are a bit out of this...and we haven't updated the wiki.
In any case (if you want to follow the wiki), I suggest you to convert topaz git repo in mercurial and use from there the recipe. Be aware that topaz integration is way back in the gem5 tree. Current gem5 is not compatible. It needs to rework the interface (its doable but not straightforward).
As last suggestion, please use the github issue track. Perhaps, some user want to do this task and can share the procedure via merge request.