From: Nsengiyumva Wilberforce <nsengiyumvawilberforce@gmail.com>

This only works for commits. Add "signature" atom with `grade`,
`signer`, `key`, `fingerprint`, `primarykeyfingerprint`, `trustlevel`
as arguments. This code and it's documentation are inspired by
how the %GG, %G?, %GS, %GK, %GF, %GP, and %GT pretty formats were
implemented.

Co-authored-by: Hariom Verma <hariom18599@gmail.com>
Co-authored-by: Jaydeep Das <jaydeepjd.8914@gmail.com>
Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Mentored-by: Hariom Verma <hariom18599@gmail.com>
Signed-off-by: Nsengiyumva Wilberforce <nsengiyumvawilberforce@gmail.com>

Submitted-As: https://lore.kernel.org/git/pull.1452.git.1672102523902.gitgitgadget@gmail.com