Release repository cache lock while materialising submodules to prevent cross-plan starvation

The shared repository cache lock is held while materialising submodules, which happens entirely within the private merge workspace and does not touch the shared cache, causing cross-plan starvation and build delays.

The lack of visibility into backend Git processing makes it difficult for both admins and users to distinguish between a hung process and a busy one.

Proposed Solution: Release the repository cache lock around submodule materialisation to eliminate cross-plan starvation without altering merge behaviour.