Teach "git tag -s" etc. a few configuration variables (gpg.format
that can be set to "openpgp" or "x509", and gpg.<format>.program
that is used to specify what program to use to deal with the format)
to allow x.509 certs with CMS via "gpgsm" to be used instead of
openpgp via "gnupg".
* hs/gpgsm:
gpg-interface t: extend the existing GPG tests with GPGSM
gpg-interface: introduce new signature format "x509" using gpgsm
gpg-interface: introduce new config to select per gpg format program
gpg-interface: do not hardcode the key string len anymore
gpg-interface: introduce an abstraction for multiple gpg formats
t/t7510: check the validation of the new config gpg.format
gpg-interface: add new config to select how to sign a commit