git diff <commit> doesn't quite work as documented?

From: Olaf Klischat <olaf.klischat@gmail.com>
Date: Sep 7, 2017, 16:31 UTC
Message-ID: <0696f94d-28c2-1f4b-03ee-16553d97f318@gmail.com>

oklischat@oklischat:/tmp$ mkdir gittest
oklischat@oklischat:/tmp$ cd gittest/
oklischat@oklischat:/tmp/gittest$ git init
Initialized empty Git repository in /private/tmp/gittest/.git/
oklischat@oklischat:/tmp/gittest$ echo foo > foo.txt
oklischat@oklischat:/tmp/gittest$ git add foo.txt
oklischat@oklischat:/tmp/gittest$ git commit -m foo
[master (root-commit) 54d55f2] foo
 1 file changed, 1 insertion(+)
 create mode 100644 foo.txt
oklischat@oklischat:/tmp/gittest$ echo bar > bar.txt
oklischat@oklischat:/tmp/gittest$ git add bar.txt
oklischat@oklischat:/tmp/gittest$ git commit -m bar
[master 83b2e74] bar
 1 file changed, 1 insertion(+)
 create mode 100644 bar.txt
oklischat@oklischat:/tmp/gittest$ git tag bar-added
oklischat@oklischat:/tmp/gittest$ git rm bar.txt
rm 'bar.txt'
oklischat@oklischat:/tmp/gittest$ git commit -m 'rm bar'
[master 3ca4ff9] rm bar
 1 file changed, 1 deletion(-)
 delete mode 100644 bar.txt
oklischat@oklischat:/tmp/gittest$
oklischat@oklischat:/tmp/gittest$ git checkout bar-added -- bar.txt
oklischat@oklischat:/tmp/gittest$ git reset HEAD
oklischat@oklischat:/tmp/gittest$ git status
On branch master
Untracked files:
  (use "git add <file>..." to include in what will be committed)

	bar.txt

nothing added to commit but untracked files present (use "git add" to track) oklischat@oklischat:/tmp/gittest$ git diff bar-added # would expect this to show no differences

diff --git a/bar.txt b/bar.txt
deleted file mode 100644
index 5716ca5..0000000
--- a/bar.txt
+++ /dev/null
@@ -1 +0,0 @@
-bar
oklischat@oklischat:/tmp/gittest$ 
oklischat@oklischat:/tmp/gittest$ git diff bar-added  -- bar.txt   # dito
diff --git a/bar.txt b/bar.txt
deleted file mode 100644
index 5716ca5..0000000
--- a/bar.txt
+++ /dev/null
@@ -1 +0,0 @@
-bar
oklischat@oklischat:/tmp/gittest$ 


`git diff --help' says:

git diff [--options] <commit> [--] [<path>...]
           This form is to view the changes you have in your working tree relative to the named <commit>.

If that were entirely true, the last two commands shouldn't have shown
any differences, right?

On closer inspection, it seems that what `git diff <commit>' really
does is take only those paths in the working directory that are also
in <commit> and compare the resulting tree against <commit>.

We should add some option to that git diff form to make it really do
what the docs claim it does.

Or am I missing something?