Add (back) 'Show Branches as Tree' toggle, for consistency and convenience

[goran-w]

It would be more consistent (and equally useful) to have a toggle-button Show Branches as Tree for LOCAL BRANCHES and for the branches under each of the top-levels under REMOTES, just like there are corresponding toggle-buttons for TAGS and SUBMODULES.

Since a "flat" view is sometimes preferred, it ought to be equally available for all these categories.

[goran-w]

I can see why this would be a bit harder (but not impossible) to implement below each "top-level" node under REMOTES.

We could start by adding the Show Branches as Tree feature for LOCAL BRANCHES, which should be simple and very similar to the implementation for TAGS.

Alos, just to clarify : the branches lists are currently always "shown as tree", but the toggle-button would allow for switching to a "flat" view (just like the corresponding feature for Tags and Submodules, respectively).

[goran-w]

Motivation:

• For hierarchical content a tree-view is often great.
• However, when you want to see the full content all at once, switching to a flat-view gives a kind of total overview that a tree-view can't.
• (An expand/collapse-all ability is another way to get a total overview, but whether that's available or not it's STILL sometimes very useful to see the full "paths" in a flat-view instead of a tree-view...)

[goran-w]

@love-linger Could we please, as a start, add (back?) the "Show branches as tree" toggle for Local Branches (at least)?

My case at hand is that all our (lots of) Remote Branches are hierarchical and I'm (usually) fine with viewing them in a tree, but for the few Local Branches I have checked-out I (most often) want to view them in a "flat" list instead. I think this choice should be up to the user, and not dictated by the UI!

NOTE: Of course, I want that toggle on Remote Branches as well, for consistency, but I realize that's slightly more difficult to implement since it would need to affect only the branch-tree/list below each Remote. And having it on local branches is a step in the right direction.