Add support for relativeWorktrees git extension

src/Microsoft.Build.Tasks.Git.UnitTests/GitRepositoryTests.cs

@@ -239,6 +239,33 @@ public void OpenRepository_Version1_Extensions()
             Assert.Null(repository.WorkingDirectory);
         }
 
+        [Fact]
+        public void OpenRepository_Version1_RelativeWorktreesExtension()
+        {
+            using var temp = new TempRoot();
+
+            var workingDir = temp.CreateDirectory();
+            var gitDir = workingDir.CreateDirectory(".git");
+
+            gitDir.CreateFile("HEAD");
+            gitDir.CreateFile("config").WriteAllText(@"
+[core]
+	repositoryformatversion = 1
+[extensions]
+    relativeWorktrees = true
+");
+
+            Assert.True(GitRepository.TryFindRepository(gitDir.Path, out var location));
+            Assert.Equal(gitDir.Path, location.CommonDirectory);
+            Assert.Equal(gitDir.Path, location.GitDirectory);
+            Assert.Null(location.WorkingDirectory);
+
+            var repository = GitRepository.OpenRepository(location, GitEnvironment.Empty);
+            Assert.Equal(gitDir.Path, repository.CommonDirectory);
+            Assert.Equal(gitDir.Path, repository.GitDirectory);
+            Assert.Null(repository.WorkingDirectory);
+        }
+
         [Fact]
         public void OpenRepository_Version1_UnknownExtension()
         {

src/Microsoft.Build.Tasks.Git/GitDataReader/GitRepository.cs

@@ -25,7 +25,7 @@ internal sealed class GitRepository
         private const string GitModulesFileName = ".gitmodules";
 
         private static readonly ImmutableArray<string> s_knownExtensions =
-            ImmutableArray.Create("noop", "preciousObjects", "partialclone", "worktreeConfig");
+            ImmutableArray.Create("noop", "preciousObjects", "partialclone", "worktreeConfig", "relativeWorktrees");
 
         public GitConfig Config { get; }