git diff：编码空间和已经commit的区别

git diff master：编码空间与远程仓库的区别

git diff origin/master或者git diff FETCH_HEAD：比较的是编码空间和远程仓库的区别

git diff origin/master master：远程仓库与本地仓库区别

git diff --cached：暂存区和最新的commit的区别

git diff [–options] --cached [] [–] […​]
This form is to view the changes you staged for the next commit relative to the named . Typically you would want comparison with the latest commit, so if you do not give , it defaults to HEAD. If HEAD does not exist (e.g. unborn branches) and is not given, it shows all staged changes. --staged is a synonym of --cached.

备注：
working space：编码空间，主要是工作区总记不住。
HEAD = master
FETCH_HEAD = origin/master