fix: the config file's directory no longer decides the rootdir alone - #14837

Split out of #14807 after review — that PR keeps the refactor and the non-breaking -c fixes, this one carries the single breaking change.

Depends on #14807 (the branch is based on it); the diff against main will shrink to one commit once that lands.

Supersedes #14454 and #14579. Closes #13246, closes #9703.

The problem

-c set the rootdir to the config file's parent directory. With the common -c config/pytest.ini layout that moved the rootdir into config/, so conftests collected an empty path prefix: same-named fixtures from sibling directories shadowed each other, and node ids came out as config::test2 rather than tests/tests2/test2.py::test2.

The change

The config file's location is evidence about where the project lives, not a decision on its own. Take the common ancestor of the config file's directory and the collected test paths, using the invocation directory in their place when no paths were given. If the two live in unrelated trees the ancestor degrades to the filesystem root, in which case the config file's directory is kept, as today.

• determine_setup: use invocation_dir as rootdir when -c is given #14454 (@EternalRights) found the root cause and drove the design discussion. Its invocation_dir rule is right for the no-args case but follows the working directory even when that is unrelated to the project. Its ambiguity warning is also written to Parser.extra_info, which is only rendered by PytestArgumentParser.error() — so it never appears on a successful run, and its tests pass by asserting on config._parser.extra_info directly.
• Fix rootdir with explicit config and test paths #14579 (@hariharan077) contributed the common-ancestor form, which is most of the answer, but special-cases if dirs and so leaves the no-args case unfixed.

No ambiguity warning is needed once the ancestor also considers the invocation directory, because the rootdir is then derived rather than guessed.

⚠️ Breaking, and which release

This changes documented behaviour — doc/en/reference/customize.rst said "If -c is passed in the command-line, use that as configuration file, and its directory as rootdir" — so the doc is updated here and the changelog entries are filed as breaking, not bugfix.

@bluetech asked on #14807 whether the next release is intended to be major. The versionchanged directive in the doc currently says 10.0 on that assumption — tell me the target and I will adjust it. If the next release is 9.2 instead, this PR should simply wait.

Concretely, what breaks: a project that today runs -c cfg/pytest.ini and relies on the rootdir being cfg/ — node ids, .pytest_cache location, and rootdir-relative ini paths all move. In practice such a setup already has broken conftest discovery, which is what #13246 and #9703 report.

Verification

Full suite green (4349 passed, 47 skipped, 13 xfailed, 7 xpassed — the xpasses are pre-existing, #11603/#10042).

Verified against the original reproducers by swapping in main's findpaths.py and running the same commands: on main the #13246 repro gives FAILED config::test2 - assert 1 == 2 and rootdir: .../config; with this commit, 2 passed and rootdir: .../ — the project root.