Avoid re-fetching merge-base SHA from Gitaly unnecessarily
This removes an unnecessary N+1 call to fetch the merge-base SHA from Gitaly by re-using the base_sha from Gitlab::Diff::Position.
Why was this MR needed?
https://gitlab.com/gitlab-org/gitlab-ce/issues/43961
What are the relevant issue numbers?
Edited by Oswaldo Ferreira