Fix commit signature check with recent versions of git

Match U for the output of "git log --format=format:%G?.

Recent versions of git will print U instead of G for good but untrusted
signatures.
timestamp: Use git-show -s option to skip diff

Instead of "| head -1"
Use a wrapper to run gpg from git

This allow us to use specific keyring files for some projects. While
doing this we add the gpg_wrapper, gpg_keyring, gpg_keyring_dir,
gpg_args, gpg_bin options.