[Git] Automatically sign-off commits (#12595)

It's very annoying when I forgot to add `-s` in `git commit` to
sign-off, because I then need to `git rebase HEAD~1 --signoff` and `git
push -f` to fix the DCO. This PR adds a hook to sign off commits
automatically when `-s` is missing to solve this problem. The only
change from the user side is now users have to install 2 hooks, so
instead of just

```
pre-commit install
```

Now we need to

```
pre-commit install --hook-type pre-commit --hook-type commit-msg
```

Note that even if users still only install the pre-commit hook, they
won't get any error in `git commit`. Just the sign-off hook won't run.

cc @hmellor @youkaichao

---------

Signed-off-by: Cody Yu <hao.yu.cody@gmail.com>

.pre-commit-config.yaml

@@ -85,9 +85,22 @@ repos:
     entry: tools/png-lint.sh
     language: script
     types: [png]
+  - id: signoff-commit
+    name: Sign-off Commit
+    entry: bash
+    args:
+      - -c
+      - |
+        if ! grep -q "^Signed-off-by: $(git config user.name) <$(git config user.email)>" .git/COMMIT_EDITMSG; then
+          printf "\nSigned-off-by: $(git config user.name) <$(git config user.email)>\n" >> .git/COMMIT_EDITMSG
+        fi
+    language: system
+    verbose: true
+    stages: [commit-msg]
   - id: suggestion
     name: Suggestion
     entry: bash -c 'echo "To bypass pre-commit hooks, add --no-verify to git commit."'
     language: system
     verbose: true
     pass_filenames: false
+