Add the commit.gpgsign option to sign all commits

If you want to GPG sign all your commits, you have to add the -S option
all the time. The commit.gpgsign config option allows to sign all
commits automatically.

Signed-off-by: Nicolas Vigier <boklm@mars-attacks.org>
test the commit.gpgsign config option

The tests are checking that :

- when commit.gpgsign is true, "git commit" creates signed commits

- when commit.gpgsign is false, "git commit" creates unsigned commits

- when commit.gpgsign is true, "git commit --no-gpg-sign" creates
  unsigned commits

- when commit.gpgsign is true, "git rebase -f" creates signed commits

Signed-off-by: Nicolas Vigier <boklm@mars-attacks.org>