Fix planning of star-schema-style queries.

Part of the intent of the parameterized-path mechanism was to handle
star-schema queries efficiently, but some overly-restrictive search
limiting logic added in commit e2fa76d8
prevented such cases from working as desired.  Fix that and add a
regression test about it.  Per gripe from Marc Cousin.

This is arguably a bug rather than a new feature, so back-patch to 9.2
where parameterized paths were introduced.