GitHub GPG sign commit

Required by some repo's for contributions.

list current gpg keys

gpg --list-secret-keys --keyid-format=long

edit key
```
gpg --edit-key <GPG key ID>
```
edit and save with
```
gpg> adduid
.... edit/update
gpg> save
```

export and upload to github

$ gpg --armor --export <<e.g.3AA5C34371567BD2>>

set the gpg key as git signing key

gpg --list-secret-keys --keyid-format=long

git config --local user.signingkey "${YOUR_SIGNING_KEY}"

git config --local commit.gpgsign true

# verify gpg and key
export GPG_TTY=$(tty)
echo "test" | gpg --clearsign

git commit -S -m "my commit msg."

git log --show-signature

5b. sign previous commit

# verify gpg and key
export GPG_TTY=$(tty)
echo "test" | gpg --clearsign

git rebase HEAD~1 --signoff --gpg-sign=52779322CDB44421

git rebase --continue