Add unit test for #1782

This unit test demonstrates the exception that was reported in #1782.

Author: ermshiperete

LibGit2Sharp.Tests/BranchFixture.cs

@@ -879,6 +879,39 @@ public void CanUnsetTrackedBranch()
             }
         }
 
+        [Fact]
+        public void CanSetTrackedBranch_MultipleFetchSpecs()
+        {
+            const string testBranchName = "master";
+            const string trackedBranchName = "refs/remotes/origin/master";
+
+            string path = SandboxMultiFetchSpecTestRepo();
+            using (var repo = new Repository(path))
+            {
+                Branch trackedBranch = repo.Branches[trackedBranchName];
+                Assert.True(trackedBranch.IsRemote);
+
+                Branch branch = repo.CreateBranch(testBranchName, trackedBranch.Tip);
+                Assert.False(branch.IsTracking);
+
+                repo.Branches.Update(branch,
+                    b => b.TrackedBranch = trackedBranch.CanonicalName);
+
+                // Verify the immutability of the branch.
+                Assert.False(branch.IsTracking);
+
+                // Get the updated branch information.
+                branch = repo.Branches[testBranchName];
+
+                Remote upstreamRemote = repo.Network.Remotes["origin"];
+                Assert.NotNull(upstreamRemote);
+
+                Assert.True(branch.IsTracking);
+                Assert.Equal(trackedBranch, branch.TrackedBranch);
+                Assert.Equal("origin", branch.RemoteName);
+            }
+        }
+
         [Fact]
         public void CanWalkCommitsFromBranch()
         {

LibGit2Sharp.Tests/Resources/multifetchspec_testrepo_wd/AnotherFile.txt

@@ -0,0 +1 @@
+This is another file

LibGit2Sharp.Tests/Resources/multifetchspec_testrepo_wd/dot_git/FETCH_HEAD

@@ -0,0 +1 @@
+ref: refs/heads/PR-28

LibGit2Sharp.Tests/Resources/multifetchspec_testrepo_wd/dot_git/HEAD

@@ -0,0 +1,14 @@
+[core]
+	repositoryformatversion = 0
+	filemode = false
+	bare = false
+	logallrefupdates = true
+	ignorecase = true
+[remote "origin"]
+	url = ../multifetchspec_testrepo.git/
+	fetch = +refs/pull/28/head:refs/remotes/origin/PR-28
+	fetch = +refs/heads/master:refs/remotes/origin/master
+	fetch = +refs/heads/*:refs/remotes/origin/*
+[branch "PR-28"]
+	remote = origin
+	merge = refs/pull/28/head

LibGit2Sharp.Tests/Resources/multifetchspec_testrepo_wd/dot_git/config

@@ -0,0 +1 @@
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