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