git archive -o something.tar.zst but file info just says "POSIX tar archive"

[Bagas Sanjaya <bagasdotme@gmail.com>]

Hi,

I noticed the following (possible bug?) when I tried to create zstd tar 
archive (.tar.zst) with `git archive`.

First, I created the plain tar archive with `git archive`, then extract 
and rearchive to zstd tar achive:

```
(on the repo)

$ git archive -o /tmp/something.tar --prefix=something/ HEAD

(outside the repo, on /tmp)

$ tar xvf something.tar
$ tar --zstd -c -v -f something.tar.zst something/
```

I checked that the archive was indeed zstd tar archive:

```
$ file something.tar.zst
something.tar.zst: Zstandard compressed data (v0.8+), Dictionary ID: None
```

Now I created the same archive with `git archive` directly:

```
(on the repo)

$ git archive -o /tmp/something1.tar.zst --prefix=something/ HEAD
```

But that archive info (with `file`) was something different:

```
(outside the repo, on /tmp)
$ file something1.tar.zst
something1.tar.zst: POSIX tar archive
```

I expected that `something1.tar.zst` was proper zstd tar archive, and 
not plain archive like above.

-- 
An old man doll... just what I always wanted! - Clara