shallow: give write_one_shallow() its own hex buffer

The previous fix reuses the local `hex` variable that is already computed at the top of `write_one_shallow()`. That works today, but `oid_to_hex()` returns a pointer into a small rotating buffer, so it is not stable across an unrelated call to `oid_to_hex()` from the same thread. A future edit that adds such a call between the assignment and the last user of `hex` would silently corrupt the output. Move `write_one_shallow()` off the rotating buffer entirely by using a local buffer instead. The current users of that `hex` variable are unchanged. Suggested-by: Junio C Hamano <gitster@pobox.com> Assisted-by: Claude Opus 4.7 Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>