Add null checks to BranchCollection

- Add various null checks to `BranchCollection`.
- Also refactor the code a bit for readability and improved stack traces.

Author: asbjornu

src/GitVersion.LibGit2Sharp/Git/BranchCollection.cs

@@ -1,23 +1,47 @@
+using GitVersion.Extensions;
+using LibGit2Sharp;
+
 namespace GitVersion;
 
 internal sealed class BranchCollection : IBranchCollection
 {
     private readonly LibGit2Sharp.BranchCollection innerCollection;
-    internal BranchCollection(LibGit2Sharp.BranchCollection collection) => this.innerCollection = collection;
 
-    public IEnumerator<IBranch> GetEnumerator() => this.innerCollection.Select(branch => new Branch(branch)).GetEnumerator();
+    internal BranchCollection(LibGit2Sharp.BranchCollection collection)
+        => this.innerCollection = collection.NotNull();
+
+    public IEnumerator<IBranch> GetEnumerator()
+        => this.innerCollection.Select(branch => new Branch(branch)).GetEnumerator();
+
     IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
+
     public IBranch? this[string name]
     {
         get
         {
+            name = name.NotNull();
             var branch = this.innerCollection[name];
             return branch is null ? null : new Branch(branch);
         }
     }
 
-    public IEnumerable<IBranch> ExcludeBranches(IEnumerable<IBranch> branchesToExclude) =>
-        this.Where(b => branchesToExclude.All(bte => !b.Equals(bte)));
-    public void UpdateTrackedBranch(IBranch branch, string remoteTrackingReferenceName) =>
-        this.innerCollection.Update((Branch)branch, b => b.TrackedBranch = remoteTrackingReferenceName);
+    public IEnumerable<IBranch> ExcludeBranches(IEnumerable<IBranch> branchesToExclude)
+    {
+        branchesToExclude = branchesToExclude.NotNull();
+
+        bool BranchIsNotExcluded(IBranch branch)
+            => branchesToExclude.All(branchToExclude => !branch.Equals(branchToExclude));
+
+        return this.Where(BranchIsNotExcluded);
+    }
+
+    public void UpdateTrackedBranch(IBranch branch, string remoteTrackingReferenceName)
+    {
+        var branchToUpdate = (Branch)branch.NotNull();
+
+        void Updater(BranchUpdater branchUpdater) =>
+            branchUpdater.TrackedBranch = remoteTrackingReferenceName;
+
+        this.innerCollection.Update(branchToUpdate, Updater);
+    }
 }