git-annex/doc/bugs/diff_driver_using_hash_without_subdir.mdwn

### Please describe the problem.
I have a strange problem with diff driver.
I wrote a diff driver which normally works, but one of my repo
it comes up with an error that it can open `.git/annex/objects/SHA256E-s4545--9715...`.
Indeed there is no such file, the file being not directly under `.git/annex/objects` but under `.git/annex/objects/mz/Px`.
Also when I examine the file in git its value is `.git/annex/objects/SHA256E-s4545--9715...` but I thought it should have been `.git/annex/objects/mz/PX/SHA256E-s4545--9715...`.


### What steps will reproduce the problem?

Just use `git diff HEAD` on my corrupted repo.


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

Version 7. I don't remember which version was used to create the repo.


### Please provide any additional information below.

[[!format sh """
# If you can, paste a complete transcript of the problem occurring here.
# If the problem is with the git-annex assistant, paste in .git/annex/daemon.log

     ❯ git diff HEAD .                                                                                                                                                                  [20:10:33]
1:dumps/pages.sql
2:/tmp/gwSIvM_pages.sql
3:1bb7ca74a6701b1ea0d4f12ae64cfc3e83e22bad
4:100644
5:dumps/pages.sql
6:0000000000000000000000000000000000000000
7:100644
-------          /tmp/gwSIvM_pages.sql-
head: cannot open '.git/annex/objects/SHA256E-s4542--9715a9ca90cad176f69fb91ed650a02afb2885c96975064d49e85230513c2d9e.sql' for reading: No such file or directory
--------         $ target
--
-- PostgreSQL database dump
--
 
-- Dumped from database version 13.3
-- Dumped by pg_dump version 13.3
 
 ❯ cat ~/.local/bin/mydiff.sh                                                                                                                                                       [20:16:07]
echo 1:$1
echo 2:$2
echo 3:$3
echo 4:$4
echo 5:$5
echo 6:$6
echo 7:$7
if [ "$5" = "/dev/null" ]
then
   echo "NULL"
   target=$1
else
   target=$5
fi
#echo $@
echo "-------          $2-"
head .git$(cat $2)
echo "--------         $target"
head $target
echo "-------- DIFF"
#colordiff $1 $2

#git --no-pager diff --color=auto --word-diff --no-ext-diff --no-index "$2" "$target"
echo diff -d --color=always $2 $target
#git --no-pager diff --color=auto --no-ext-diff --no-index -U0 "$2" "$5"


# End of transcript or log.
"""]]

### Have you had any luck using git-annex before? (Sometimes we get tired of reading bug reports all day and a lil' positive end note does wonders)
Yes and it works really well (and my diff driver works on other repos too).


> [[notabug|done]]