Bug in git archive + .gitattributes + relative path

[Cristian Le <cristian.le@mpsd.mpg.de>]

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Using `git archive` with or without `--worktree-attributes` does not 
properly read `.gitattributes` files if using a relative path in 
`<tree-ish>`.
Related github comment: 
https://github.com/rpm-software-management/tito/pull/445#issuecomment-1450298871
Related stackoverflow discussion: 
https://stackoverflow.com/questions/52804334/how-to-ignore-files-directories-in-git-archive-and-only-create-an-archive-of-a

Git version: `2.39.2`
Mwe git repo: Two files:
```
# .gitattributes:
.git_archival.txt export-subst
```
```
# .git_archival.txt:
node: $Format:%H$
```

Commands to reproduce and expected behaviour:
```console
$ git archive HEAD:./ --output=test.tar.gz
$ tar -axf test.tar.gz .git_archival.txt -O
node: 745ce26169fb44e04d91d40ee581cccd591c941e
```
Important: Notice the path `./` given after `HEAD`.

Actual output:
```console
$ tar -axf test.tar.gz .git_archival.txt -O
node: $Format:%H$
```

It doesn't matter if `.gitattributes` is in a subfolder, or if I change 
the relative path `./` to a subfolder, the files are still not properly 
generated.

Using `--worktree-attributes` did not have any effect either.
According to the documentation, I understand that the expected behaviour 
with regards to `--worktree-attributes`:
- Read the `.gitattributes` of the relative path, e.g. `./sub_dir` 
regardless of `--worktree-attributes`. (similar behaviour as not passing 
a relative path)
- Include the `.gitattributes` of the top-level path if 
`--worktree-attributes` is passed

Maybe the intended behaviour is to completely ignore all 
`.gitattributes` unless `--worktree-attributes` is provided, in which 
case, it does not have the intended behaviour and please include a flag 
to achieve the above behaviour.


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