Fix how setup.py identifies the Mercurial version

[Jeremy Whitlock]

> Thanks for looking into this!

No problem.

> I get the same error after deleting all the .pyc files -- the
> traceback is from the 'hg' spawned by setup.py and results in the
> version being reported as unknown in the built 'hg'.

Do you get the problem without my patch or do you also see it after
the patch is applied?  I downloaded the 1.2.1 release tarball, applied
my patch and it ran fine so I'm a little confused if the problem still
persists after my patch is applied.

> Mercurial can be run without building the C extensions, but it
> requires fiddling with the PYTHONPATH in order to bring the modules in
> mercurial/pure into play.

Yeah.  I saw that but I didn't see it being handled properly in
setup.py so I went this route.  Appears to be pretty solid and low
risk but I'll leave the review up to you and others.  :)

-- 
Take care,

Jeremy Whitlock
http://www.thoughtspark.org