Add to tree button in repository tree header doesn't have tooltip or aria-label for accessibility

Summary

Add to tree button + in repository tree header is not screen-reader friendly, while it is a dropdown and underlying options have title, but the button itself is missing tooltip or aria-label.

Steps to reproduce

I used tota11y by Khan Academy for identifying accessibility issues. Instructions to install and use it are provided on project homepage that I have linked.

Expected behavior

Screen-reader should be able to identify button as a dropdown with futher options.

Actual behavior

Screen-reader doesn't identify the button.

Screenshots