Enable nilnil linter for new code (#36591)

Fixes: https://github.com/go-gitea/gitea/issues/36152

Enable the `nilnil` linter while adding `//nolint` comments to existing
violations. This will ensure no new issues enter the code base while we
can fix existing issues gradually.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

services/pull/check.go

@@ -291,7 +291,7 @@ func getMergeCommit(ctx context.Context, pr *issues_model.PullRequest) (*git.Com
 	if err := gitrepo.RunCmdWithStderr(ctx, pr.BaseRepo, cmd); err != nil {
 		if gitcmd.IsErrorExitCode(err, 1) {
 			// prHeadRef is not an ancestor of the base branch
-			return nil, nil
+			return nil, nil //nolint:nilnil // return nil to indicate that the PR head is not merged
 		}
 		// Errors are signaled by a non-zero status that is not 1
 		return nil, fmt.Errorf("%-v git merge-base --is-ancestor: %w", pr, err)