git-blame-ignore-revs: Add previous commit

And enable use of the auto-rebase script: https://github.com/NixOS/nixpkgs/tree/master/maintainers/scripts/auto-rebase

Author: infinisil

.git-blame-ignore-revs

@@ -241,3 +241,6 @@ e0fe216f4912dd88a021d12a44155fd2cfeb31c8
 
 # nixos/iso-image.nix: nixfmt
 da9a092c34cef6947d7aee2b134f61df45171631
+
+# treewide format of all Nix files
+6ea5fa88a2a4f929d16b2d7ea923af3feadf091f # !autorebase nix-shell --run treefmt