Update script should detect when files have been edited in the local repo

[jleibs]

The update script will blindly overwrite local edits for any files not in the per-language allow-lists.

There are many things missing from these lists that are still likely to be edited. For example:

• .gitignore
• lychee config
• typos config

It would be nice if we specifically detected these files don't match the history from the template repo and subsequently prompt the user about changing them, or maybe even try to apply as patch.