SCM Graph - Add a `Collapse All` action

The expand feature could also benefit by a collapse all button in case user wants to collapse all the expanded commit (this is just my personal preference)

Originally posted by @sanket-bhalerao in #238339

When "workbench.tree.expandMode": "singleClick" (the default) each time you click on a row in the graph it expands to show the files involved in the commit.

Keyboard shortcut Ctrl+LeftArrow will collapse all expanded rows, but the user might not be aware of this.

I suggest we add a Collapse All action, defaulting it to appear in the overflow menu of the Graph view but with the option of moving it to be a button on the toolbar.