[BUGFIX] Azdo client fetches wrong branch's SHA

TargetBranchCommitSha was erroneously called "target branch" because it
represents the branch in the codeflow that belongs to the `target` repo.
Howevever, in practice it is actually the PR Head branch. Therefore it
is now renamed to HeadBranchCommitSha.
Furthermore, setting HeadBRanchCommitSha is fixed in the Azdo client.

Author: adamzip

src/Microsoft.DotNet.Darc/DarcLib/AzureDevOpsClient.cs

@@ -344,7 +344,7 @@ public async Task<PullRequest> GetPullRequestAsync(string pullRequestUrl)
                 _ => PrStatus.None,
             },
             UpdatedAt = DateTimeOffset.UtcNow,
-            TargetBranchCommitSha = pr.LastMergeTargetCommit.CommitId,
+            HeadBranchCommitSha = pr.LastMergeSourceCommit.CommitId,
         };
     }
 

src/Microsoft.DotNet.Darc/DarcLib/Models/PullRequest.cs

@@ -12,5 +12,5 @@ public class PullRequest
     public string HeadBranch { get; set; }
     public PrStatus Status { get; set; }
     public DateTimeOffset UpdatedAt { get; set; }
-    public string TargetBranchCommitSha { get; set; }
+    public string HeadBranchCommitSha { get; set; }
 }

src/ProductConstructionService/ProductConstructionService.DependencyFlow/PullRequestUpdater.cs

@@ -414,11 +414,11 @@ private async Task<MergePolicyCheckResult> TryMergingPrAsync(
         PullRequestUpdateSummary prSummary = CreatePrSummaryFromInProgressPr(pr, targetRepository);
         MergePolicyEvaluationResults? cachedResults = await _mergePolicyEvaluationState.TryGetStateAsync();
 
-        IEnumerable<MergePolicyEvaluationResult> updatedMergePolicyResults = await _mergePolicyEvaluator.EvaluateAsync(prSummary, remote, policyDefinitions, cachedResults, prInfo.TargetBranchCommitSha);
+        IEnumerable<MergePolicyEvaluationResult> updatedMergePolicyResults = await _mergePolicyEvaluator.EvaluateAsync(prSummary, remote, policyDefinitions, cachedResults, prInfo.HeadBranchCommitSha);
 
         MergePolicyEvaluationResults updatedResult = new MergePolicyEvaluationResults(
             updatedMergePolicyResults.ToImmutableList(),
-            prInfo.TargetBranchCommitSha);
+            prInfo.HeadBranchCommitSha);
 
         await _mergePolicyEvaluationState.SetAsync(updatedResult);
 

test/ProductConstructionService.DependencyFlow.Tests/PullRequestUpdaterTests.cs

@@ -466,7 +466,7 @@ protected IDisposable WithExistingCodeFlowPullRequest(
                 Status = prStatus,
                 HeadBranch = InProgressPrHeadBranch,
                 BaseBranch = TargetBranch,
-                TargetBranchCommitSha = "sha123456",
+                HeadBranchCommitSha = "sha123456",
             });
 
         if (willFlowNewBuild