.gitignore 文件修改后，repo 内包含和舍弃文件的效果并不会立即兑现，需要用 git 命令行进行一些操作。
Ignore files that have already been committed to a Git repository
To untrack a single file that has already been added/initialized to your repository, i.e., stop tracking the file but not delete it from your system use: git rm --cached filename
To untrack every file that is now in your .gitignore:
First commit any outstanding code changes, and then, run this command:
git rm -r --cached .
This removes any changed files from the index(staging area), then just run:
git add .
git commit -m ".gitignore is now working"