Normalize git commondir path.

The constructor for GitRepositoryLocation asserts that this path is
normalized. On my machine, a git worktree created by git 2.22.0 has a
commondir file that contains "../..". This results in path that does not
satisfy this assert.

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

@@ -58,6 +58,56 @@ public void TryFindRepository_Worktree()
             Assert.Null(location.WorkingDirectory);
         }
 
+        [Fact]
+        public void TryFindRepository_Worktree_Realistic()
+        {
+            using var temp = new TempRoot();
+
+            var mainWorkingDir = temp.CreateDirectory();
+            var mainWorkingSubDir = mainWorkingDir.CreateDirectory("A");
+            var mainGitDir = mainWorkingDir.CreateDirectory(".git");
+            mainGitDir.CreateFile("HEAD");
+
+            var worktreesDir = mainGitDir.CreateDirectory("worktrees");
+            var worktreeGitDir = worktreesDir.CreateDirectory("myworktree");
+            var worktreeGitSubDir = worktreeGitDir.CreateDirectory("B");
+            var worktreeDir = temp.CreateDirectory();
+            var worktreeSubDir = worktreeDir.CreateDirectory("C");
+            var worktreeGitFile = worktreeDir.CreateFile(".git").WriteAllText("gitdir: " + worktreeGitDir + " \r\n\t\v");
+
+            worktreeGitDir.CreateFile("HEAD");
+            worktreeGitDir.CreateFile("commondir").WriteAllText("../..\n");
+            worktreeGitDir.CreateFile("gitdir").WriteAllText(worktreeGitFile.Path + " \r\n\t\v");
+
+            // start under main repository directory:
+            Assert.True(GitRepository.TryFindRepository(mainWorkingSubDir.Path, out var location));
+
+            Assert.Equal(mainGitDir.Path, location.GitDirectory);
+            Assert.Equal(mainGitDir.Path, location.CommonDirectory);
+            Assert.Equal(mainWorkingDir.Path, location.WorkingDirectory);
+
+            // start at main git directory (git config works from this dir, but git status requires work dir):
+            Assert.True(GitRepository.TryFindRepository(mainGitDir.Path, out location));
+
+            Assert.Equal(mainGitDir.Path, location.GitDirectory);
+            Assert.Equal(mainGitDir.Path, location.CommonDirectory);
+            Assert.Null(location.WorkingDirectory);
+
+            // start under worktree directory:
+            Assert.True(GitRepository.TryFindRepository(worktreeSubDir.Path, out location));
+
+            Assert.Equal(worktreeGitDir.Path, location.GitDirectory);
+            Assert.Equal(mainGitDir.Path, location.CommonDirectory);
+            Assert.Equal(worktreeDir.Path, location.WorkingDirectory);
+
+            // start under worktree git directory (git config works from this dir, but git status requires work dir):
+            Assert.True(GitRepository.TryFindRepository(worktreeGitSubDir.Path, out location));
+
+            Assert.Equal(worktreeGitDir.Path, location.GitDirectory);
+            Assert.Equal(mainGitDir.Path, location.CommonDirectory);
+            Assert.Null(location.WorkingDirectory);
+        }
+
         [Fact]
         public void LocateRepository_Submodule()
         {

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

@@ -491,6 +491,8 @@ private static bool IsGitDirectory(string directory, out string commonDirectory)
                 try
                 {
                     commonDirectory = Path.Combine(directory, File.ReadAllText(commonLinkPath).TrimEnd(CharUtils.AsciiWhitespace));
+                    // Normalize relative paths. For example, git worktrees typically have "../.." in this file.
+                    commonDirectory = Path.GetFullPath(commonDirectory);
                 }
                 catch
                 {