Add a commit.signOff config feature

[Denton-L]

When committing, this should act like --signoff is given on the command-line (unless --no-signoff is given).

Notes:

• we have to document --no-signoff in commit
• do we other commands to learn this as well? (rebase.signOff? merge.signOff?)

[Denton-L]

Implementation here: https://lore.kernel.org/git/20200105174127.9278-1-hji@dyntopia.com/T/#m6b7b2afa3df5bbf5a7c10fc171fb0507115d1b74

[Denton-L]

This will probably never be implemented: https://lore.kernel.org/git/xmqqpnfw8gyn.fsf@gitster-ct.c.googlers.com/