[Feature flag] Rollout of `lfs_link_existing_object`

What

The lfs_link_existing_object feature flag has been reintroduced in !65437 (merged)

Before enabling it, let's fix the underlying bug: #331364 (closed)

Owners

• Team: Source Code
• Most appropriate slack channel to reach out to: #g_create_source-code
• Best individual to reach out to: @igor.drozdov