Investigate and reduce Gitaly N+1 calls on diff_stats GraphQL field
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## Background
Based on investigation done on https://gitlab.com/groups/gitlab-org/-/epics/12441#note_1730085427, the `diff_stats` field in `GetProjectMergeRequests` query can result to Gitaly N+1 calls for `DiffStats` RPC.
## Proposal
Investigate and reduce the Gitaly N+1 calls when `diff_stats` is queried.
issue