In my case, this was caused by using the same Git repo from both admin and non-admin command prompts. When last git pull was from admin cmd, the index was created by it, and then non-admin cmd had insufficient permissions to modify it.
My solution was re-creating the index (while keeping the worktree intact):
del .git\index
git reset --mixed head
自己尝试的方案
遇到的问题是，使用git status会报这个错误
因为git bash是直接安装到环境变量的，用管理员权限打开powershell，然后在powershell里面执行git status就不会报错了。