* gh: option to enable helper for additional hosts
`gh` can also be used with github enterprise
hosts, for which there exists no easy option
to enable the credential helper except for
directly working with `programs.git.extraConfig`.
Not sure if this is a needed addition since it's
somewhat niche, at the same time it's not very
complex and makes the life of github enterprise
a little easier.
* gh: update credential-helper tests
* gh: refactor credential helper option
this moves from `enableGitCredentialHelper` to
`gitCredentialHelper.enable` and
`gitCredentialHelper.hosts`.
* gh: lib.mkIf -> mkIf
