### Please describe the problem.

[[!format sh """
(git-annex)lena:/tmp/testds3[master]git-annex
$> git diff HEAD^                            
--- file.dat      2021-04-08 13:38:57.578593717 -0400
+++ /home/yoh/.tmp/hT81L0_file.dat        2021-04-08 13:39:04.794565306 -0400
@@ -1 +1 @@
-4
+/annex/objects/MD5E-s2--6d7fce9fee471194aa8b5b6e47267f03.dat
"""]]

NB edit: order of the diff is reverted from what it should be, I guess need to swap `$1` and `$2` in the driver

<details>
<summary>my current git config and (dirty) git-annex diffdriver</summary> 

```shell

$> git config  diff.external                                                          
git-annex diffdriver -- git-annex-diff-wrapper --

$> cat ~/bin/git-annex-diff-wrapper 
#!/usr/bin/env bash                                                             
#set -eu
echo "Called with $@ under $PWD" >> /tmp/calls
LANG=C colordiff --color=auto -u "$1" "$2"                                                   
exit 0
#LANG=C diff --color=always -u "$2" "$5"                                                   
#LANG=C diff --color=always --ignore-matching-lines="\(AFNI version=\|time.*Elapsed\)" -u "$2" "$5"                                                   
#label="$1:${2##*/}->${5##*/}" 
label="$1"
regex="XXXXXX" #\(AFNI version=\|time.*Elapsed\)"
LANG=C diff --label "$1" --color=auto --ignore-matching-lines="$regex" -u "$2" "$5"                                                   
exit 0

```
</details>

<details>
<summary>a more complete dump from the terminal</summary> 

```shell
(git-annex)lena:/tmp/testds3[master]git-annex
$> rm  file.dat 
(dev3) 1 70670 [3].....................................:Thu 08 Apr 2021 01:38:54 PM EDT:.
(git-annex)lena:/tmp/testds3[master]git-annex
$> echo 4 >| file.dat
(dev3) 1 70671 [3].....................................:Thu 08 Apr 2021 01:38:57 PM EDT:.
(git-annex)lena:/tmp/testds3[master]git-annex
$> git commit -m 'commit unlocked 4' file.dat
[master 5f57c78] commit unlocked 4
 1 file changed, 1 insertion(+), 1 deletion(-)
(dev3) 1 70672 [3].....................................:Thu 08 Apr 2021 01:39:02 PM EDT:.
(git-annex)lena:/tmp/testds3[master]git-annex
$> git diff HEAD^                            
--- file.dat      2021-04-08 13:38:57.578593717 -0400
+++ /home/yoh/.tmp/hT81L0_file.dat        2021-04-08 13:39:04.794565306 -0400
@@ -1 +1 @@
-4
+/annex/objects/MD5E-s2--6d7fce9fee471194aa8b5b6e47267f03.dat
(dev3) 1 70673 [3].....................................:Thu 08 Apr 2021 01:39:04 PM EDT:.
(git-annex)lena:/tmp/testds3[master]git-annex
$> cat file.dat 
4
(dev3) 1 70674 [3].....................................:Thu 08 Apr 2021 01:39:17 PM EDT:.
(git-annex)lena:/tmp/testds3[master]git-annex
$> git --version
git version 2.29.2
(dev3) 1 70675 [3].....................................:Thu 08 Apr 2021 01:39:24 PM EDT:.
(git-annex)lena:/tmp/testds3[master]git-annex
$> git annex version | head -n 1
git-annex version: 8.20210223

```
</details>

### What steps will reproduce the problem?

<details>
<summary>a complete but dirty (uses datalad) shell history</summary> 

```shell
datalad create /tmp/testds3
cd /tmp/testds3
echo 1 > file.dat
datalad save -m 1
rm file.dat
echo 2 > file.dat
datalad save -m 2
git diff HEAD^^
git diff HEAD^
git annex unlock file.dat
git add file.dat
echo 3 >| file.dat
git commit -m 'commit unlocked 3'
git commit --amend
git commit -m 'commit unlocked 3' file.dat
git diff HEAD^
git diff HEAD^^
rm file.dat
echo 4 >| file.dat
git commit -m 'commit unlocked 4' file.dat
git diff HEAD^
cat file.dat
git --version
git annex version | head -n 1
git status

```
</details>

### What version of git-annex are you using? On what operating system?


8.20210223

[[!meta author=yoh]]
[[!tag projects/datalad]]

> [[fixed|done]] --[[Joey]]
