ENH: Add compare URL when updating extension

Change SlicerWizard.py to load the old extension description when
submitting an update to an extension, and to use this to generate and
include a compare URL in the pull request message with the changes in
the extension from the currently available (indexed) version.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22971 3bd1e089-480b-0410-8dfb-8563597acbee