Provide visual indications of remotes' prune-able branches names in the sidebar
The problem
Oftentimes, individuals' and projects' remote repositories use temporary branches that get deleted after merge (hopefully). You can typically do git prune the_remote
to get all of those stale branches removed from your view, making things clearer and speeding up gitg. However, gitg does not give you visual hints that it might be possible and timely to do so.
Proposed solution
I believe those "deleted upstream" branches' names should have their rows/text be made 50% opacity (or maybe 70% opacity) in the sidebar's treeview, and when the mouse hovers them, they should show a GtkTooltip that says something like:
This branch has been deleted from the "%s" remote repository, and can be pruned from your local copy.
\n\n
Right-click the "%s" remote to prune it.
...that way, it would be much more obvious.