Allow a Git-common-dir-relative data directory as an opt-in fallback for ephemeral environments

[eitsupi]

It would be useful to configure the roborev data directory per local Git clone, without changing .roborev.toml.

My use case is an ephemeral Dev Container: $HOME may disappear when the container is recreated, while the Git repository persists.
I would like to keep the reviews.db inside the repository's local Git metadata instead.

One possible design:

git config --local roborev.dataDir roborev

• Relative paths are resolved against the Git common directory, e.g. <common-dir>/roborev.
• Linked worktrees therefore share the same roborev database.
• ROBOREV_DATA_DIR remains the explicit override.
• If neither is set, the existing ~/.roborev default remains unchanged.

roborev already exposes CheckoutMetadata.CommonDir, so this seems compatible with the existing worktree handling.