Add end-to-end test for make pgxntool-sync

[jnasbyupgrade]

Context

update-setup-files.sh has thorough unit tests (test/standard/update-setup-files.bats in pgxntool-test) that test 3-way merge scenarios in isolation. But there is no test for the full make pgxntool-sync flow: git subtree pull → update-setup-files.sh.

This test is needed before adding subdirectory support (#TBD), since that work will modify the sync target's -P prefix logic.

Approach

Create test/standard/pgxntool-sync.bats in pgxntool-test that:

1. Creates a local, temporary git repo inside the test's $TEST_DIR as a pgxntool source (this is NOT a third GitHub repo — it's a throwaway repo that gets cleaned up with the test environment). This is needed because git subtree pull requires pulling from a real git repo, and the actual pgxntool repo may be dirty during test runs.

2. Tags two versions in that temp repo (v1 = baseline, v2 = adds a marker line to _.gitignore)

3. Creates a test extension repo using git subtree add from v1

4. Runs make pgxntool-sync-test to pull v2

5. Verifies:

   • pgxntool files reflect v2 after sync
   • .gitignore was auto-updated by update-setup-files.sh
   • A merge commit was created

How pgxntool-sync works

pgxntool-sync-%:
	@old_commit=$$(git log -1 --format=%H -- pgxntool/) && \
	git subtree pull -P pgxntool --squash -m "Pull pgxntool from $($@)" $($@) && \
	pgxntool/update-setup-files.sh "$$old_commit"

The % pattern is looked up as a make variable, e.g.:

• make pgxntool-sync-local → $(pgxntool-sync-local) = ../pgxntool master
• Custom: make pgxntool-sync-foo 'pgxntool-sync-foo=/path/to/repo branch'

The test uses the custom variable approach to point at the local temp repo.