Undo all changes since opening buffer in vim

You can use the

:edit!

command to get into the earliest saved state. See :help edit! for more information.

You can also check something like gundo.vim (can be found here), which displays the whole undo tree graphically, and you can easily jump between points. Then there is the histwin plugin which I did not used yet, but offers similar functionality.

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

:u1|u

The first command (:undo 1) reverts to the state of the buffer just 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 accepts the change number 0 as a valid argument to the :undo command, finally providing a way to refer to the state prior to any registered changes. This makes it possible to achieve the same effect in a single-command invocation:

:u0

In vim 8.1+ as well as in neovim, you can just use :u0