如何让暂存区的文件恢复到和HEAD一样以及如何让工作区的文件恢复到和暂存区一样

$ git status #将文件修改好加入暂存区，查看当前状态
On branch master
Changes to be committed:
  (use "git reset HEAD <file>..." to unstage)

	modified:   a
	modified:   abc
$ git reset HEAD #根据提示 使用reset HEAD命令将所有暂存区文件都还原到HEAD
Unstaged changes after reset:
M	a
M	abc
$ git status #OK，还原成功
On branch master
Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git checkout -- <file>..." to discard changes in working directory)

	modified:   a
	modified:   abc

no changes added to commit (use "git add" and/or "git commit -a")
$ git diff --cached #还可以通过这种方式查看暂存区有没有文件和HEAD存在差异，没有返回证明无差异

$ git status #查看当前目录状态 有三个已经加入暂存区的文件
On branch master
Changes to be committed:
  (use "git reset HEAD <file>..." to unstage)

	modified:   abc
	modified:   add
	modified:   commit1

$ git reset HEAD -- abc commit1 #使用reset命令只还原abc和commit1文件
Unstaged changes after reset:
M	abc
M	commit1
$ git status #再次查看目录状态 abc和commit1文件已经从暂存区中被取消
On branch master
Changes to be committed:
  (use "git reset HEAD <file>..." to unstage)

	modified:   add

Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git checkout -- <file>..." to discard changes in working directory)

	modified:   abc
	modified:   commit1

$ vi a
$ git add a #假设修改了a文件 并将a加入暂存区
$ vi a #然后觉得a不太好，又修改了内容
$ git diff #修改后查看了工作区和暂存区的差异 发现工作区的修改还不如暂存区
diff --git a/a b/a
index 79ca77c..e63ef0c 100644
--- a/a
+++ b/a
@@ -1 +1 @@
-Hello,I'm a!!!
+Hello,I'm a!!!.....
$ git status #查看一下当前状态
On branch master
Changes to be committed:
  (use "git reset HEAD <file>..." to unstage)

	modified:   a

Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git checkout -- <file>..." to discard changes in working directory)

	modified:   a
$ git checkout -- a #按照上边的提示 忽略对a文件的修改
$ git status #再次查看状态 工作区的修改已经被抹掉
On branch master
Changes to be committed:
  (use "git reset HEAD <file>..." to unstage)

	modified:   a