git-crypt fails to add-gpg-user when gpg.program is set to gpgsm

[btdonovan]

Git-crypt fails to find users in the public keyring when git.program is set to gpgsm because gpgsm certs are prefixed with crt instead of pub. gpgsm also lacks a --fixed-list-mode option but that style output is the default in gpgsm so it could be safely dropped if gpgsm is the git.program.

Fixing these two problems would enable git-crypt to make use of x509 certificates for encryption.