您不能仅通过WorkItemHttpClient来获得提交注释，可以与GitHttpClient一起获得。首先，使用WorkItemHttpClient获取工作项链接，然后使用GitHttpClient获取提交ID和评论。
一个工作示例：
VssClientCredentials cred = new VssClientCredentials();
VssConnection tfs = new VssConnection(new Uri("http://tfs-server:8080/tfs/collection"),cred);
var workItemClient = tfs.GetClient<WorkItemTrackingHttpClient>();
var gitClient = tfs.GetClient<GitHttpClient>();
int workItemId = 1213;
var workItem = workItemClient.GetWorkItemAsync("Project-Name",workItemId,expand: WorkItemExpand.Relations).Result;
// We need to retrieve the commit id from the links,debug the following line to understand what I did
var commitId = wit.Relations.Where(r => r.Url.Contains("Git")).FirstOrDefault().Url.Split('%')[2].Remove(0,2);
var commit = gitClient.GetCommitAsync("Project-Name",commitId,"Repo-Name").Result;
string comment = commit.comment;
顺便说一句，您不能使用Fixes #123语法，因为TFS 2017不支持该语法。