17比较工作区和暂存区所含文件的差异

场景

修改仓库文件提交到暂存区后还未commit，此时再次对仓库文件进行修改，比较修改后工作目录文件和暂存区文件差异。

git diff: 后面不加具体文件则默认比较工作区和暂存区所有文件的区别

git diff -- [具体文件1] [具体文件2]: 查看工作区和暂存区中具体文件的变动

 G:\mygitea\GitLearn\learn03   master ± 
$ git diff
diff --git a/readme b/readme
index edd02cd..4dd1fab 100644
--- a/readme
+++ b/readme
@@ -4,3 +4,4 @@ This is the young branch.
 after test, update readme.
 test HEAD and cached compare fix 2.
 2
+123445
diff --git a/test.txt b/test.txt
index f29dee6..4debe7e 100644
--- a/test.txt
+++ b/test.txt
@@ -1 +1,2 @@
 测试
+222

 G:\mygitea\GitLearn\learn03   master ± 
$ git diff -- readme
diff --git a/readme b/readme
index edd02cd..4dd1fab 100644
--- a/readme
+++ b/readme
@@ -4,3 +4,4 @@ This is the young branch.
 after test, update readme.
 test HEAD and cached compare fix 2.
 2
+123445

总结

假定：HEAD、缓存区、工作区中的readme.md文件内容均不相同。
git diff HEAD -- readme.md      # 工作区 <===> HEAD
git diff -- readme.md           # 工作区 <===> 缓存区
git diff --cached -- readme.md  # 缓存区 <===> HEAD

Q1:Git diff — file. 这里的—是什么意思，没有也能正常工作？