The current conversion process mandates an empty repository for a clean start.
This presents a barrier to performance optimization strategies.
This change introduces the ability to pass a repository root commit hash.
This is necessary to support the immediate next commit (Incremental LFS conversion),
which uses a `.gitattributes` file and LFS pointers to bypass the slow, full-history
rewriting often required on large non-empty monorepos (100GiB+, 1M+ files).
The immediate benefit is allowing conversion to start when a non-empty repo
already contains an orphan commit, laying the groundwork for the optimized LFS
conversion feature.
