httprepo: quote the path part of the URL
This should fix a 'hg clone "http://hg.example.org/path with spaces/"'
The code tries to do the right thing when the user passes a path that's
already escaped in part (e.g. "http://hg.example.org/path%20with spaces/").
If we're redirected, urllib2 will happily follow the URL it's given
without escaping anything. I'm not sure what we would have to hook
to work around that.