Filtering Script Should Have Option to Check On Disk File

[dsanders11]

Add an option to the filtering script to filter out changes which are already present on disk. This will be most useful for the header mapping feature, since that's done in the filtering script and there may be changes which can be filtered out post-mapping. Also useful for pulling an include changes CSV more up-to-date without needing to re-run suggest_include_changes.py on a newer revision.