Stopping extdiff from deleting the temp files it creates?

[Jens Alfke]

I want my GUI app to be able to use "hg extdiff" to invoke a separate  
GUI diff application (FileMerge, a built-in OS X app.) The problem is  
that extdiff deletes the temporary files it creates, after the diff  
command returns -- and and the tool that opens FileMerge returns right  
after the window opens. That means that, if comparing directories, you  
can't actually view any of the file diffs, because the files you click  
on have already been deleted.

Is there a way to tell extdiff to not delete its temporary files?

--Jens