Use shared/tmp/rebase instead of tmp/rebase

This issue tracks the migration from using tmp/rebase to shared/tmp/rebase for better shared storage compatibility in GitLab installations.

Background

The current implementation uses tmp/rebase which can cause issues in environments with shared storage or when multiple GitLab instances are running.

Proposed Solution

Migrate to using shared/tmp/rebase directory structure to:

• Improve compatibility with shared storage setups
• Reduce conflicts between multiple GitLab instances
• Follow established patterns for shared temporary directories

Implementation Details

• Update rebase logic to use shared temporary directory
• Ensure proper cleanup of temporary files
• Maintain backward compatibility during transition
• Update relevant documentation