Stacked PR checkouts can lack `origin/master`, causing `bazel_run_codeowners` to fail during changed-file discovery even though Evergreen already passes `--expansions-file`.

Update `_discover()` in `buildscripts/quality_checks/cli.py` to resolve the default diff base through `evergreen_git.get_diff_revision()` when an expansions file is supplied. Explicit `-origin-branch`, `-all`, and file scopes retain their behavior. Add regression coverage in `quality_checks_test.py` using real Git repositories without remote refs for base PRs, stacked PRs, CLI patches, and waterfall builds.

Fixes SERVER-136643(https://jira.mongodb.org/browse/SERVER-136643).

Validation: all 79 quality-check and wrapper unit tests pass; `bazel run format` and `git diff --check` pass. Evergreen CI has not been run.