Only check LFS integrity for first branch in push

What

Limits lfs_objects_exist_check to one change per push

EE counterpart in gitlab-org/gitlab-ee!4547

Why

We call ChangeAccess once for each branch in a push. When doing a git push --all this can be too much for a slow check such as this one.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• End-to-end tests pass (package-qa manual pipeline job)

Related

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/41141