Use `gpg.program` in Git config

[pittma]

Currently, use of gpg is hardcoded. Instead, it ought to check whether a user has the config key gpg.program set, and use it if so. If not, it can fall back on a default.

[pittma]

This came up with a coworker whose gpg and gpg2 are distinct versions and the hardcoded gpg is giving them trouble. It's something I could take on, but not for a couple of weeks. If someone else wants to take it on in the meantime, I'm happy to help with any guidance needed!