This switches GitClient to use run_process() instead of the legacy

execute(). This ensures that every call site is deliberate in the

kinds of data it's expected to work with and how errors are handled.
The call sites have been audited to ensure that we're only redirecting

stderr to stdout when we should and that we're explicitly handling error

conditions. This may not be perfect, but it's already fixed up a few

places where we may have been doing things wrong before.
All git config calls are now handled by a _get_git_config(), which

offers a standardized way of fetching configuration data, returning the

string if found, None if not found, or raising SCMError if we get a

fatal: result (such as if running outside of a Git tree).

Unit tests pass.
Posted this change for review.