Bug: undotree split window width behave in a weird way with nvim-tree

[Mccranky83]

Undotree works in tandem with nvim-tree if the diff pane is toggled off.

When nvim-tree closes, the width of undotree remains the same.

However, if the diff pane is toggled on before nvim-tree first opens, when nvim-tree eventually closes, the space occupied by it will be allocated to undotree, causing it to have a wider span.