Ignore "format the world" commit in git blame

This tells github to hide it in its blame view: https://docs.github.com/en/repositories/working-with-files/using-files/viewing-a-file#ignore-commits-in-the-blame-view
It can also be used locally by running `git config blame.ignorerevsfile .git-blame-ignore-revs` (although it's advised to avoid `--global` since git gives a hard error when the file doesn't exist).

We may want to add more commits in later PRs, but this should be a good start.

Author: jyn514

.git-blame-ignore-revs

@@ -0,0 +1,6 @@
+# format the world
+a06baa56b95674fc626b3c3fd680d6a65357fe60
+# format libcore
+95e00bfed801e264e9c4ac817004153ca0f19eb6
+# reformat with new rustfmt
+971c549ca334b7b7406e61e958efcca9c4152822