Add a `.git-blame-ignore-revs` file (#23959)
## Summary
This should make it easier to spelunk through the `git blame` of a file
on GitHub. I believe you can also configure git locally to respect this
file as well when spelunking through blame, though I've never tried that
## Test Plan
Don't know, but I've copied the pattern I've seen e.g. in
https://github.com/python/mypy/blob/master/.git-blame-ignore-revs, which
I believe has been successful!
