add support for maintaining git history for both sides of the codemodded files

[void-mAlex]

create a branch for renaming both parts of the combined file to cause a conflict and show git which parts come from which file

[davidtaylorhq]

I took a stab at implementing this technique in a standalone script, so you just pass it the base branch (e.g. main) and your codemodded branch. Very hacky so far, but it does seem to work: https://gist.github.com/davidtaylorhq/484505beccd5eeac1166bd75002b3e5a

Here's an example of the resultant commits: https://github.com/discourse/discourse/commits/codemod-squashed-example

And an example blame of a codemodded component: https://github.com/discourse/discourse/blame/codemod-squashed-example/app/assets/javascripts/discourse/app/components/avatar-uploader.gjs

(adding the codemod commit to .git-blame-ignore-revs was important, because otherwise the indentation change was showing in the blame for the entire template)

[ef4]

Yes, we discussed that to really land this is should be a standalone command closer to that.

That history looks nice on GitHub. I didn't realize that GitHub supports .git-blame-ignore-revs by default.