Add missing .gitignore files.

Tom Lane <tgl@sss.pgh.pa.us>

Commit: f7da81f68b63430d24caba99c0a95933c10cefcb
Author: Tom Lane <tgl@sss.pgh.pa.us>
Date: 2026-04-06T17:25:29Z
Add missing .gitignore files.

contrib/pg_stash_advice and src/test/modules/test_shmem
missed these, leading to complaints from git after an
in-tree check-world run.

Use our standard boilerplate list of ignorable subdirectories,
although the two modules presently create different subsets
of that.

Files