set_hgexecutable() on windows

[Andrei Vermel]

> The new set_hgexecutable() stuff in 0.9.4 doesn't work for me on
> Windows XP; my HG environment variable always ends up set to how I
> invoke Mercurial from the command line.

I have a similar problem with hg installed from source, which I call with a 
batch file hg.cmd containing 'python path-to-hg/hg'
In my case $HG gets set to 'path-to-hg/hg', and it can't be exec'ed from hgk
apparently.

Andrei