Checksum calculation does not include default branch
From https://gitlab.com/gitlab-org/gitlab-ee/issues/7211 and https://gitlab.com/gitlab-com/production/issues/408, we found that the checksum calculation omits the HEAD
entry from the calculation.
That's because we are calling git show-ref --heads --tags
when we should be calling git show-ref --head --tags
.
We'll need to fix https://gitlab.com/gitlab-org/gitlab-ee/issues/7211 with this, since the verification will always mismatch until we can update the local HEAD.