Is your feature request related to a problem? Please describe.
When longer branch names are used, the branch name gets truncated very quickly in the legend panel's branch button, and it lacks a tooltip to show the branch's full name.

Describe the solution you'd like
Please notice how the commit's SHA button has a tooltipped tooltipped-sw CSS class assigned, which displays a custom "white-on-black" tooltip, with the tooltip's content taken from the aria-label="...." attribute of that tooltipped element.

All this requires is adding the aria-label="${branchName}" attribute (source of the tooltip's content) and adding the class="... tooltipped tooltipped-sw" CSS classes to the element.

It should be possible to replicate this same functionality for the branch button pretty easily, maybe even switching the direction of the tooltip to e.g. north (tooltipped tooltipped-n, as we have blank space there anyway), please see the DevTools-hacked example below:

Describe alternatives you've considered
Adding a standard tooltip="${branchName}" attribute might be an alternative, but the tooltipped custom CSS tooltip results in a more fancy-looking tooltip appearance.