[RFE] Add config option to always add --signoff to git commit

Hello, I would lide to propose an enhancement.

As a user, I need several repositories to have my commits both gpg signed, and their messages decorated with the `Signed-off-by:` trailer. This is a requirement to have the contribution accepted by respective communities.

The first part is easy to achieve using `commit.gpgsign=true`, but the latter, not so much.

Caveats with current feature set:

- Passing `--signoff`/`-s` every time is easy to forget. Even more so 
when some commits can be done by external tools (IDEs doing git pull 
with merge, etc.)

- Creating Git alias almost get there, but one cannot override what `git 
commit` does. So one have to remember to use the alias, that is error prone.

- Using custom hooks to pre-prepare the message is a bit more elaborate. 
Scales poorly with growing number of repositories requiring signoff. Can 
collide with other hooks touching the message for some other reason.

Possible courses of action:

- Introduce new config option `commit.signoff`, that would be 
independent of `commit.gpgsign`. It would simply cause `git commit` to 
always behave like `git commit --signoff`.

- Add 3rd possible value to `commit.gpgsign` to both sign and add trailer.

- Change `commit.gpgsign` semantics to always add the `Signed-off-by:` 
trailer if set to true. Likely have compatibility implications.

- Train developers to never forget `--signoff`/`-s`, befriend `git 
rebase --signoff` :-/

I appreciate any feedback. Thanks!