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