Add .git-blame-ignore-revs file

Add the file to allow everyone to ignore certain revs when looking at
git blame. Setting this up requires running

```
git config blame.ignoreRevsFile .git-blame-ignore-revs
```

once on the repo after clone.

.git-blame-ignore-revs

@@ -0,0 +1,12 @@
+# Since version 2.23 (released in August 2019), git-blame has a feature
+# to ignore or bypass certain commits.
+#
+# This file contains a list of commits that are not likely what you
+# are looking for in a blame, such as mass reformatting or renaming.
+# You can set this file as a default ignore file for blame by running
+# the following command.
+#
+# $ git config blame.ignoreRevsFile .git-blame-ignore-revs
+
+# 2.14.8-9-gba9680f: Use Black code formatter
+ba9680fbdeebc0e91d481518424d2c2ed5c92112