Adjust test-docker release branch check for Jenkins quirk

[riftEmber]

Fix the check in util/cron/test-docker.bash that ensures we're working from the desired release branch when in release mode.

When run in the Jenkins job, this check failed despite having the correct revision checked out. This is because for some reason Jenkins (or the SCM plugin we use, or JJB, not sure) takes the branch you ask to check out, manually dereferences it to the commit it points to, and then checks out that commit by SHA. So HEAD points to a SHA rather than the branch name, and git rev-parse --abbrev-ref HEAD just returns HEAD.

Fixed by comparing the SHA of HEAD against the SHA of the desired branch directly.

[trivial, not reviewed]

Testing:

• manual run of test-docker.bash in release mode