Fix bug related to size check of repositories and files

What does this MR do?

It changes the strategy to calculate the size of the changes received in the push. Now we use a new RPC created in Gitaly that will only return the new blobs that were added or modified by the current push.

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

Why was this MR needed?

Some users were unable to push a new branch/tag or when they were pushing a branch with a single change we were counting the size of all the existing files in the repo.

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
• Conforms to the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• EE specific content should be in the top level /ee folder
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #6459 (closed)

Closes #6780 (closed)