A GitHub rebase merge seems to rewrite commits even if it would be a
fast-forward, which means that the commit merged from #24718 is wrong.
- [x] update `.git-blame-ignore-revs` with real commit from `develop`
- [x] add `.git-blame-ignore-revs` to ignore black reformatting
- [x] make `spack blame` respect `.git-blame-ignore-revs`
(even if the user hasn't configured git to do so)