Tooltip of divergence-graph always assumes master as the default branch

Summary

The tooltip of the divergence-graph in the branches view always read like

xxx commits behind master, xxx commits ahead

However, if 'master' is not the default branch this is wrong.

Steps to reproduce

Select a branch not named 'master' as default branch then go to the branches view and inspect the tooltips of the divergence-graphs.

What is the current bug behavior?

The tooltip always prints 'master'.

What is the expected correct behavior?

The tooltip should print the name of the default branch instead.