STYLE: Add debugging to Extension Wizard

Add additional debugging information to the Extension Wizard when
creating a compare URL for a pull request to (hopefully) help diagnose
failures. Add a (hidden) --dryRun option which will skip actually
creating the pull request during --contribute.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23254 3bd1e089-480b-0410-8dfb-8563597acbee