Script for diff'ing with upstream geth

[gakonst]

Due to the monorepo flow, it is now hard to diff the l2geth with upstream geth from inside the repository. We should have a script which allows giving a path to a local geth (or a remote geth tag) and prints out the diff from current version.

[gakonst]

Updated idea from discussion with @snario:

• Git should version control a .diff file (a patch on geth which contains the OVM changes)
• yarn download-geth should fetch geth at some specific version and apply the patch
• Whenever there is a new geth version some dependabot will auto create a new pull request with the naive merge strategy
• If it just works, merge it. Otherwise maybe assign someone to deal with the merge conflict

This will allow us to always be up-to-date with each geth version, instead of having to manually rebase.

[smartcontracts]

Going to close this for two reasons:

1. It's not too hard to diff with upstream Geth, simply git diff ./go-ethereum ./optimism/l2geth. I do this quite frequently when looking for ways to reduce the diff.
2. As part of Bedrock, we're going to be moving away from l2geth in the monorepo in favor of having geth in a separate repository, mainly for the purpose of maintaining history more easily. This will effectively fix the issue.