有趣的问题！我调查了一下，发现如下：
git有两个独立的zsh补全系统：

1. git（https://github.com/git/git/blob/master/contrib/completion/git-completion.zsh）附带的一个，如the docs you linked所示，它基于任何可用的git二进制文件提供了你所拥有的补全系统。
   1.与zsh（https://github.com/zsh-users/zsh/blob/master/Completion/Unix/Command/_git）一起发布的版本要复杂得多，而且显然更能感知上下文。
   使用一个空的zshrc（zsh -d -f -i）启动一个新的zsh会话，我们会看到：

josh% autoload -Uz compinit && compinit
josh% git che<TAB>
check-attr       -- display gitattributes information
check-ignore     -- debug gitignore/exclude files
check-mailmap    -- show canonical names and email addresses of contacts
check-ref-format -- ensure that a reference name is well formed
checkout         -- checkout branch or paths to working tree
checkout-index   -- copy files from index to working directory
cherry           -- find commits not merged upstream
cherry-pick      -- apply changes introduced by some existing commits

然后，在加载其他完井系统后（我将在下面讨论如何准确地执行此操作）：

josh% git che<TAB>

变成

josh% git checkout

那么我们如何切换呢？我的方法是通过OMZ插件gitfast。我实际上并不（但愿不会）使用OMZ，而是克隆插件并将其源代码导入到我的fpath中的zshrc和fpath=( $XDG_CONFIG_HOME/ohmyzsh/plugins/gitfast $fpath )中
但看起来更直接的方法是按照该repo中_git的说明操作：

# The recommended way to install this script is to make a copy of it as a
# file named '_git' inside any directory in your fpath.
#
# For example, create a directory '~/.zsh/', copy this file to '~/.zsh/_git',
# and then add the following to your ~/.zshrc file:
#
#  fpath=(~/.zsh $fpath)
#
# You need git's bash completion script installed. By default bash-completion's
# location will be used (e.g. pkg-config --variable=completionsdir bash-completion).
#
# If your bash completion script is somewhere else, you can specify the
# location in your ~/.zshrc:
#
#  zstyle ':completion:*:*:git:*' script ~/.git-completion.bash