Reference transaction hook is unable to find the old_oid for pre-existing refs

[elhmn <elhmn42@xxxxxxxxx>]

While inspecting the reference transaction hook code,
I realised that the hook was receiving a CREATE ref update line even for
references that already existed on disk, where I was expecting to see
an UPDATE ref update line.
That behaviour was observed for the file and the reftable backends.

Using the `reference-transaction` file bellow:
```
$ cat .git/hooks/reference-transaction
if test "$1" = prepared
then
         while read -r line
         do
                 printf "%s\n" "$line"
         done >>actual
fi
```

I created the commits A and B

```
$ git commit --allow-empty -m "A"; git tag A
$ git commit --allow-empty -m "B"; git tag B
```

Checked which refs were created:
```
$ git show-ref
9b18557013105bb7a7bf681f18757084ada9d948 refs/heads/master
01077b2840db5baea0084921d8f3158a240e8d85 refs/tags/A
9b18557013105bb7a7bf681f18757084ada9d948 refs/tags/B
```

Then ran `git update-ref refs/heads/master A`  which logged:
```
0000000000000000000000000000000000000000
01077b2840db5baea0084921d8f3158a240e8d85 refs/heads/master
```

But I expected:
```
9b18557013105bb7a7bf681f18757084ada9d948
01077b2840db5baea0084921d8f3158a240e8d85 refs/heads/master
```

The git/git t1416-ref-transaction-hooks.sh update tests seem to be
expecting a $ZERO_OID for reference update, so
I am not sure if that's a bug or a feature.

A test case: https://github.com/git/git/blob/2462961280690837670d997bde64bd4ebf8ae66d/t/t1416-ref-transaction-hooks.sh#L55

CREATE: <zero_oid> <new_oid> <ref_name>
UPDATE: <old_oid> <new_oid> <ref_name>

I ran those tests using the git version `2.51.0` on `Ubuntu 20.04.6 LTS`.