This adds regression testing for the fix in 6c0cc71 (GitoxideLabs#2111).

The `packed_refs_are_looked_up_when_checking_existing_values` test
case had already asserted that the updated ref is loose and parses
as the correct OID. This extends that case with another assertion
that the stored file's exact contents are the hex OID followed by
a newline.

The added assertion checks the OID, and not just that the file
ends with a newline character, partly so that if it somehow opened
the wrong file then this would be caught, but more importantly so
that it verifies that no extra data beyond this are added. For
example, a loose ref should end with a Unix-style LF line ending
on all systems; by checking its whole content, this verifies that
no CR (`\r`) byte is written even on Windows.

The new assertion was validated by applying it locally at fb78f1e,
where it fails in the expected way:

            FAIL [   0.560s] gix-ref-tests::refs file::transaction::prepare_and_commit::create_or_update::packed_refs_are_looked_up_when_checking_existing_values
      stdout ───

        running 1 test
        test file::transaction::prepare_and_commit::create_or_update::packed_refs_are_looked_up_when_checking_existing_values ... FAILED

        failures:

        failures:
            file::transaction::prepare_and_commit::create_or_update::packed_refs_are_looked_up_when_checking_existing_values

        test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 134 filtered out; finished in 0.52s

      stderr ───

        thread 'file::transaction::prepare_and_commit::create_or_update::packed_refs_are_looked_up_when_checking_existing_values' panicked at gix-ref\tests\refs\file\transaction\prepare_and_commit\create_or_update\mod.rs:758:5:
        assertion `left == right` failed: the loose ref is stored on disk as the full OID followed by a newline
          left: "0000000000000000000000000000000000000001"
         right: "0000000000000000000000000000000000000001\n"
        note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

In contrast, when applied at dc7343c, it passes, due to 6c0cc71.