shed_diff should...

• Build actual tar balls and diff those (so ignored files like .shed.yml are not included).
• Have a default enabled mode like that does an actual XML diff of TS artifacts and filters TS generated attribute changes - disable with --raw.

After these changes shed_diff will be ready for realized repositories and will be able to predict if a shed_upload will be needed. After that, shed-upload should diff before uploading.

This would completely alleviate my lingering concerns on galaxyproject/tools-iuc#50.