src/postgres code style guide

Everything under src/postgres is a fork of upstream PostgreSQL that gets re-merged on every PG major-version upgrade. The PG 11->15 merge hit 600+ conflicts (~2.3 per work day for a year), and every conflict avoided is catchup time saved. Every rule here serves one of two goals:

1. Keep the YB delta small, clustered, and visibly marked so a merger (human or git) can tell YB code from PG-owned code at a glance.
2. Keep PG-owned lines byte-identical to upstream so git auto-merges them and conflicts appear only where a real logical conflict exists.

The pinned upstream version of any file is defined by src/lint/upstream_repositories.csv. To see the YB delta of a file, or to fetch the pinned upstream copy itself: