When reverting via the GUI, the branch to revert from always defaults to master, even when the commit doesn't exist there

As raised in Slack by @sytses, when reverting via the GUI the branch dropdown always defaults to master, even when the commit doesn't exist on master.

Marking this as workflowproblem validation because I'm not sure what would be involved in changing this. I think this falls under backend but would need more investigation.

/cc @m_gill