BUG: ExtensionWizard: Fix "compare URL" generation

The extensionWizard now handles the creation of the "Compare URL" when someone
contributes an updated Extension Description File.

The issue appeared when using the ExtensionWizard as reported below:

$ bin/slicerExtensionWizard --contribute ~/src/Slicer-MyExtension/

The reported error was:

//------------
building compare URL for update failed to build
compare URL: 404 {u'documentation_url':
u'https://developer.github.com/v3/repos/commits/#compare-two-commits',
u'message': u'Not Found'}
//------------

Reviewed-by: Jean-Christophe Fillion-Robin <jchris.fillionr@kitware.com>
Tested-by: Jean-Christophe Fillion-Robin <jchris.fillionr@kitware.com>

From: Jean-Baptiste VIMORT <jb.vimort@gmail.com>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24667 3bd1e089-480b-0410-8dfb-8563597acbee