当执行命令
git rm -f filename
删除一个文件时出现一个错误：
fatal: Unable to create '/.git/index.lock': File exists.
If no other git process is currently running, this probably means a
git process crashed in this repository earlier. Make sure no other git
process is running and remove the file manually to continue.
解决方法：
使用命令 git rm 或者手动删除文件/.git/index.lock
