Bitbucket Compare: Dropdown button to switch the branches have missing label

Issue Description

In the "Compare" branch section, the dropdown button for switching the branch have missing labels for the button.

Steps to Reproduce

1. Launch URL
2. Navigate to the Compare link in the left sidebar section
3. Navigate to the Source in the main content area where branch lists are present.

Screenshot

Actual Behaviour

In the compare main content area, for switching the branches a dropdown list is available for the functionality. The dropdown button has a missing label, which will not indicate to the assistive technology users the functionality associated with the button. Since the button has a dropdown functionality it also has a role menu missing.

Expected Result

The dropdown button should have a unique label. In the absence of labels, it becomes difficult for users especially screen reader users to understand the purpose of the form control and input information.

An aria-label="current_branch_name" could be given to the active branch selected.

Additionally, a role="menu" to the main element and role="menuitems" could be given to the list of branches.

An example could be found at: https://www.w3.org/TR/wai-aria-practices-1.1/examples/listbox/listbox-collapsible.html

AXQA Ref:AXQA-314