How to undo all changes in a buffer made since it was open in Vim?

Question

How can I undo all changes since opening a buffer? I imagine there may be some form of :earlier that does this.

Update: Many are suggesting

Answer 1

To revert the current buffer to the state prior to the very first change recorded in the Vim undo list (see :help undo-tree), we can use two consecutive invocations of the :undo command:

:u1|u

The first command (:undo 1) reverts to the state of the buffer after the very first registered change, while the second command (:undo) reverts that first change itself.

Starting with version 8.1 (see :helpg Patch 8.0.1441), Vim allows to pass the change number 0 as an argument to the :undo command to refer to the state before any registered changes, thus making it possible to achieve the same effect in a single-command invocation:

:u0