change editor when rebase collapse or histedit is called

[Uwe Brauer]

Hi

When I call for example 
hg rebase --keep -d 2 -s 3 --collapse

The zile editor (lightweight and nice) pops up. But recently I prefer to
use GNU emacs/emacsclient. I googled around I cannot find where to
configure the editor.

Any ideas?

Thanks

Uwe Brauer