[buildbot] Fix ref master to be full ref name

Summary:
Since the Teamcity upgrade, builds using the default master ref
have been soft-failing (build completes successfully, but Teamcity fails it) with this error:

The build was triggered in the branch master which does not correspond to any branch monitored by the build VCS roots (the branch is either closed or excluded / not matched by any branch specification). Because of that default branch revisions were set to this build.

The only evidence I can find is that the ref name is now considered incorrect, whereas older versions
of Teamcity treated master and refs/heads/master as the same.

Test Plan: pytest

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7827