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>
