Bump gitlab_git to speedup DiffCollection iterations

What does this MR do?

Bump gitlab_git to v10.4.2 that incorporate some improvements on DiffCollection iterations

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

What are the relevant issue numbers?

Relates to #14775 (moved) #19451 (moved) #13912 (closed) #2529 (closed)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• [ ] Documentation created/updated
• [ ] API support added
• Tests
  • [ ] Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together