[PATCH 0 of 1] Honoring the *_proxy variables

[renato at renatocunha.com]

Hi there,

this is an attempt at better handling the *_proxy variables when working with
subrepos. This patch is admitedly incomplete, as it lacks a proper test case,
but I'd like to know if the general approach is OK.

My main idea is to only clear the http_proxy and related environment variables
*if* there's a corresponding entry in the configuration. It seems to me that
this approach is good, and it has worked in the few tests I made, BTW.

Thanks.