Move the logic to select the desired branch to its own static method.

Author: Jericho

src/GitVersionCore/GitVersionContext.cs

@@ -10,9 +10,7 @@
     public class GitVersionContext
     {
         public GitVersionContext(IRepository repository, string targetBranch, Config configuration, bool onlyEvaluateTrackedBranches = true, string commitId = null)
-             : this(repository,
-                   repository.Branches.SingleOrDefault(b => b.CanonicalName == targetBranch || b.FriendlyName == targetBranch) ?? repository.Head,
-                   configuration, onlyEvaluateTrackedBranches, commitId)
+             : this(repository, GitVersionContext.GetTargetBranch(repository, targetBranch), configuration, onlyEvaluateTrackedBranches, commitId)
         {
         }
 
@@ -148,5 +146,31 @@ void CalculateEffectiveConfiguration()
                 currentBranchConfig.TracksReleaseBranches.Value,
                 currentBranchConfig.IsReleaseBranch.Value);
         }
+
+        private static Branch GetTargetBranch(IRepository repository, string targetBranch)
+        {
+            // By default, we assume HEAD is pointing to the desired branch
+            var desiredBranch = repository.Head;
+
+            // Make sure the desired branch has been specified
+            if (!string.IsNullOrEmpty(targetBranch))
+            {
+                // There are some edge cases where HEAD is not pointing to the desired branch.
+                // Therefore it's important to verify if 'currentBranch' is indeed the desired branch.
+                if (desiredBranch.CanonicalName != targetBranch)
+                {
+                    // In the case where HEAD is not the desired branch, try to find the branch with matching name
+                    desiredBranch = repository?.Branches?
+                        .SingleOrDefault(b =>
+                            b.CanonicalName == targetBranch ||
+                            b.FriendlyName == targetBranch);
+
+                    // Failsafe in case the specified branch is invalid
+                    desiredBranch = desiredBranch ?? repository.Head;
+                }
+            }
+
+            return desiredBranch;
+        }
     }
 }