If source branch of open pull request is updated then RepositoryMirrorSynchronized event includes 3 refs:
actual branch name
it leads to several problems:
actual branch name is decorated by pr from branch , which makes difficult to distinguish invocations
refs/pull-requests/<id>/merge is in list, but actually it's not synchronized.
So required fixes for mirror events:
do not decorate regular branch with pull request branch name
exclude merge pr branch