Add a list of hashes to ignore during git blame. (#9560)

The hashes are from commits due to auto-formatting, e.g. running black.

git can be configured to use this automatically by running the following:

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

.git-blame-ignore-revs

@@ -0,0 +1,8 @@
+# Black reformatting (#5482).
+32e7c9e7f20b57dd081023ac42d6931a8da9b3a3
+
+# Target Python 3.5 with black (#8664).
+aff1eb7c671b0a3813407321d2702ec46c71fa56
+
+# Update black to 20.8b1 (#9381).
+0a00b7ff14890987f09112a2ae696c61001e6cf1

changelog.d/9560.misc

@@ -0,0 +1 @@
+Add a `.git-blame-ignore-revs` file with the hashes of auto-formatting.

setup.cfg

@@ -3,6 +3,7 @@ test_suite = tests
 
 [check-manifest]
 ignore =
+    .git-blame-ignore-revs
     contrib
     contrib/*
     docs/*