Allow specifying `gnupg home` for signed commits

[clearydude]

Currently, the git methods around signing commits do not allow for specifying a specific gnupg path. This is fine for how the end user ends up using it, since when it is not specified a specific path it finds it from the environment, but it requires tests using gnupg to use mutexes.
The gpg method it uses does allow for specifying a gnupg home dir, so if the signature was altered to enable that specification to be passed into the git method, it may mitigate the need for some of those mutexes.