Keep header visible when the view is scrolled

[t-gehring]

Describe the improvement that you'd like
Currently the graph header, containing fetch/refresh/etc buttons and the column header row, is part of the scroll area, and disappears when the graph is scrolled to view older commits. If this header had a fixed position relative to the window, buttons could be used and columns resized, even when the graph has been scrolled down.

[mhutchie]

Thanks for this suggestion!

I've been wanting to add this for a while now, but I just haven't got around to it as I've been implementing feature requests / improvements from the Git Graph user community before my own, and no one had suggested it up until now.

Since you've suggested it, this can now be scheduled for an upcoming release!

[bendera]

PR has been added.