Fix buffering in DiffCollection

What does this MR do?

Gitlab::Git::DiffCollection had a bug where if you iterated partially and then again, the second iterating run would be missing some entries. So for example if you would call #empty? and then #to_a to get an Array, the array would be missing one element (namely the second element returned by the underlying iterator).

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

Why was this MR needed?

It's not good if files disappear from a diff.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?