External diff on top-level menu doesn't work when comparing git remote branch

See https://answers.atlassian.com/questions/52375/external-diff-doesn-t-work-when-comparing-a-local-repository-vs-a-remote?page=1#52419

When you compare a git remote branch with the current version and then use the main menu Actions > External Diff, nothing happens. The button in the diff panel works.