Provide On-demand checkout way in GitLab Merge Request

### Problem Statement

Currently we only checkout 1 depth in code review job to reduce gitlab server stress like this
```
ai_code_review:
stage: verify
variables:
GIT_DEPTH: "1"
allow_failure: true
timeout: 15m
```

But ocr requires all codes between merge base to head sha exists.

### Proposed Solution

I hope there's a way to let ocr fetch the required commit on-demand so I don't need to set a high GIT_DEPTH value

### Alternatives Considered

_No response_

### Affected Area

CLI / Commands

### Additional Context

_No response_

Start with the CLI/Commands area and trace how GitLab merge-request reviews currently use the one-commit checkout described in the issue. Verify the behavior with a shallow checkout containing only the merge request head, then confirm that the review can fetch the required commits on demand without increasing GIT_DEPTH.

Written by the indexing model from the issue text.