Request: Extra case for %G? format

Hello all,

Could the %G? format differentiate between an unsigned commit and a signed commit that you're missing a public key for?

If `git show --format=%GG --no-patch <commit>' produces an output like the following:

gpg: Signature made <date> using RSA key ID <id> gpg: Can't check signature: public key not found

Then currently %G? results in `N', the same as an unsigned commit.

In this case, could %G? please result in a new character? Perhaps `M' for "missing public key"?