Add .git-blame-ignore-revs and some instructions for clang-format

.git-blame-ignore-revs

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+# _clang_format added
+19d98d6a74079a42bcc5cb0e899d239a4bdb6e20

README.clang-format

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+We introduced clang-format mandatory usage in July 2020.
+If you want git blame to ignore the revision in which we did the mass change you can do
+   git config blame.ignoreRevsFile .git-blame-ignore-revs
+on your clone
+
+To get the clang-format warnings locally instead at CI time we recommend you
+to copy the hooks/pre-commit to your .git
+    cp hooks/pre-commit .git/hooks/
+
+We are using clang-format 10 on CI. Unfortunately clang-format is not totally
+compatible with older versions of itself. If CI gives you trouble but your local
+clang-format disagrees, just apply the changes suggested by CI and then commit
+with the --no-verify flag. If you get stuck, don't hesitate to ask the reviewer
+to help and they will reformat your commits :)

hooks/pre-commit

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+#!/usr/bin/env bash
+
+readonly output=$(git clang-format -v --diff)
+
+if [[ "$output" == *"no modified files to format"* ]]; then exit 0; fi
+if [[ "$output" == *"clang-format did not modify any files"* ]]; then exit 0; fi
+
+echo "ERROR: you need to run git clang-format on your commit"
+echo "       git clang-format -f is potentially what you want"
+exit 1