Subzero: Fix SUBZERO_REVISION in production builds.

In production autoconf builds (which are the builds that matter), the SUBZERO_REVISION was being calculated based on git history of the top-level native_client repo, rather than the pnacl-subzero repo.

BUG= none
R=jpp@chromium.org, kschimpf@google.com

Review URL: https://codereview.chromium.org/2338943003 .