Mark pure reformatting PRs in .git-blame-ignore-revs

[jakkdl]

This is mostly relevant to @CoolCat467 as you're in the progress of cleaning up the repository, but I realize we in general haven't been great at keeping that file updated.

See discussion in #2686 and #2692

It would be great if one could have a check that warned about this ... but I have no clue how that would work in any way, and I'm also not sure if it's possible to learn the commit ID's before merging such that you can actually add a PR to the file before it gets merged.