Unify and use Git username/email/GPG handling.

[MikeMcQuaid]

We're using essentially the same logic to setup Git for committing in multiple places but the way we're doing so is inconsistent. Moved to using two shared utility methods and use them consistently.

Without this brew pr-pull failed consistently due to being unable to find gpg when it ran brew bottle.

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew tests with your changes locally?
• Have you successfully run brew man locally and committed any changes?

---