🐛 Use GitLab MR diff head SHA for CI statuses

[rylena]

Summary

• query GitLab merge requests for diffHeadSha
• use the diff head SHA as the associated merge request head SHA so CI status lookups target the MR head instead of the merge commit
• avoid indexing empty merge commit SHAs when GitLab returns no merge commit

Fixes #3701

Verification

• go test ./clients/gitlabrepo
• git diff --check

[bilaldeveloper4312]

Reviewed commit 364afa82 against the behavior described in #3701.

Using diffHeadSha for AssociatedMergeRequest.HeadSHA matches the required semantics: CI-Tests needs the merge-request head as it existed before merge, while mergeCommitSha can be absent for fast-forward merges and identifies a different commit when present. Guarding the merge-commit lookup also prevents an empty SHA from becoming a map key. The regression test exercises the important distinction between the MR diff head and merge commit. I found no blocking issue in the changed code.

Focused verification passes locally with Go 1.25.6:

go test ./clients/gitlabrepo -run '^(TestZipUsesDiffHeadShaForMergeRequestHead|TestListRawCommits)$' -count=1

The full package run reaches the unchanged Windows-specific TestExtractTarball/Basic path-separator and file-lock failure, so I am not representing the entire package as passing in this environment.

[rylena]

Updated this branch with the latest upstream/main to clear the behind state. Verification after the merge: go test ./clients/gitlabrepo -run '^(TestZipUsesDiffHeadShaForMergeRequestHead|TestListRawCommits)$' -count=1 and git diff --check HEAD~1..HEAD.