Add basic title popup to graphs on hover to see branch name

[phil294]

Describe the feature that you'd like

When you scroll down, the branch names disappear, and there is no way to see it, other than to scroll up to the top again and remember the color mapping. I find this annoying and would appreciate any fix to it. The most obvious one I can think of is adding a popup when you hover the branch line, showing you the name:

This would greatly improve the usability of the extension. Other solutions might be sticky branch name headers, branch name display on commit hover, branch name display on commit expand, etc

Additional context (optional) #270 suggests more sophisticated visualization for a clicked branch. While this would be good too, it seems complicated, and I find this issue to be pressing enough to justify a minimum effort transitional hover feature.

Thanks for this extension either way! It is the best out there and it works very well.

[frylo0]

@mhutchie Really need this feature. Then repository has a lot of branches become really hard to understand where your branch...