migration: Fix identifier gap in rewriter

Created by: efritz

Partial fix to #39863 (closed). This rewriter function incorrectly chose the parent of a renamed migration (an explicitly handled edge case) to be the wrong number on a number of patch versions (not tested by the stitch utility directly). This fixes the misnaming of that parent in this condition.

Test plan

Existing unit tests (caught a bug on my first try of this!). Tested explicit patch version by hand.