add .git-blame-ignore-revs to hide formatting changes in git blame (#419)

See
https://docs.github.com/en/repositories/working-with-files/using-files/viewing-a-file#ignore-commits-in-the-blame-view

Note: it only works for `git blame` (or GitHub blame page) but does not
work for `git history` (or GitHub file history page).

Author: njzjz

.git-blame-ignore-revs

@@ -0,0 +1,7 @@
+# .git-blame-ignore-revs
+# pre-commit
+ffa52c5d6230303d6f7ee4f1356f01aa5b2a011d
+# pre-commit docs python block
+ecdf566626c4b2d1824b946d1b7ad809cb8946dd
+# pre-commit imports
+4f1a91167b58f9042ae17cae05b609b7fdf5f20c