[3.13] gh-119999: Fix potential race condition in `_Py_ExplicitMergeRefcount` (GH-120000) (#120073)

We need to write to `ob_ref_local` and `ob_tid` before `ob_ref_shared`.
Once we mark `ob_ref_shared` as merged, some other thread may free the
object because the caller also passes in `-1` as `extra` to give up its
only reference.
(cherry picked from commit 4055577221f5f52af329e87f31d81bb8fb02c504)

Co-authored-by: Sam Gross <colesbury@gmail.com>