Add support for caching repositories with --reference

[staticfloat]

We should do what the buildkite agent does with the "git mirrors" experiment (which we generally have turned on).

Broadly, we should do:

REPO_URL="${BUILDKITE_PLUGIN_EXTERNAL_BUILDKITE_EXTERNAL_REPO}"
REPO_URL_HASH="$(shasum -a 256 <<<"${REPO_URL}")"
REFERENCE_REPO_PATH="/cache/external-buildkite/repos/${REPO_URL_HASH}"

# Clone our persistent mirror repository
if [[ ! -d "${REFERENCE_REPO_PATH}" ]]; then
    git clone --mirror "${REPO_URL}" "${REFERENCE_REPO_PATH}"
else
    git fetch -C "${REFERENCE_REPO_PATH}" origin
fi

# Clone a fresh checkout, using our mirror as a reference (this should generally never require network access)
git clone --reference "${REFERENCE_REPO_PATH}" "${REPO_URL}" ...