Better mechanism to choose the default editor (and avoid vi if possible)?

[Marcus Harnisch]

Is it safe to assume that users who are disturbed by this default 
behaviour are likely to work in a desktop session? In that case we could 
make use of XDG, no? I just tried ‘xdg-open ~/.config/hg/hgrc’ and it 
popped up the configured plain text editor.

Hg is already inspecting XDG_CONFIG_HOME (another XDG facility) for 
locating the configuration file.

Cheers,
Marcus