Document how to connect to a Mercurial, Git, Bitbucket or Github source repositories via a http or https proxy

Mercurial does not respect the "-Dhttp.proxyPort" property that is passed to Bamboo server or agent, as we are forking the actual hg native binary. It runs in a separate process and it is not written in Java.

The right way to configure hg to use a proxy is to set the http_proxy.host, and potentially http_proxy.user and http_proxy.password environment variables:
http://www.markhneedham.com/blog/2009/05/13/mercurial-pulling-from-behind-a-proxy/

For https there is also a https_proxy as environment variable.

We should document what should be done to access Mercurial via a proxy.

If using the Bitbucket repository in Bamboo to connect to Bitbucket (rather a simple Mercurial repository, which could also be used to connect to Bitbucket) one also needs to set the -Dhttp.proxyHost and -Dhttp.proxyPort parameters of Bamboo server. Thsi will work once BAM-10287 is fixed.

This needs to be mentioned, and linked to a more generic doc which I hope we already have about how to make Bamboo access the web through a proxy.

We also need to do some research and document how to get Git and GitHub repository to work over a proxy.