将当前缓冲区恢复到Vim撤消列表中记录的第一个
更改之前的状态（请参见：help undo-tree $ ），我们
可以使用：undo 命令的两个连续调用：

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

第一个命令（：undo 1 ）在第一个已注册的更改
之后恢复到缓冲区的状态，而第二个命令（：undo ）
会自动还原第一个更改。

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.

从8.1版开始（请参见：helpg补丁8.0.1441 ），Vim允许$​​ b $ b传递更改数字0作为：undo 命令的参数
在进行任何已更改之前引用状态，从而使
可以达到相同的效果一次调用即可完成：

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 invocation:

:u0