Properly get project root directory

This returns the correct workdir when using worktrees where the git
repo resides inside of the main working tree. Partial fix for #1063.

src/GitVersionCore.Tests/ExecuteCoreTests.cs

@@ -7,6 +7,7 @@
 using System;
 using System.IO;
 using System.Text;
+using LibGit2Sharp;
 
 [TestFixture]
 [Parallelizable(ParallelScope.None)]
@@ -266,6 +267,33 @@ public void WorkingDirectoryWithoutGit()
         });
     }
 
+    [Test]
+    [Category("NoMono")]
+    [Description("LibGit2Sharp fails when running under Mono")]
+    public void WorkingDirectoryWithWorktree()
+    {
+        var versionAndBranchFinder = new ExecuteCore(fileSystem);
+
+        RepositoryScope(versionAndBranchFinder, (fixture, vv) =>
+        {
+            var worktreePath = Path.Combine(Directory.GetParent(fixture.RepositoryPath).FullName, Guid.NewGuid().ToString());
+            try
+            {
+                // create a branch and a new worktree for it
+                var repo = new Repository(fixture.RepositoryPath);
+                repo.Worktrees.Add("worktree", worktreePath, false);
+
+                var targetUrl = "https://github.com/GitTools/GitVersion.git";
+                var gitPreparer = new GitPreparer(targetUrl, null, new Authentication(), false, worktreePath);
+                gitPreparer.GetProjectRootDirectory().TrimEnd('/', '\\').ShouldBe(worktreePath);
+            }
+            finally
+            {
+                DirectoryHelper.DeleteDirectory(worktreePath);
+            }
+        });
+    }
+
     [Test]
     public void DynamicRepositoriesShouldNotErrorWithFailedToFindGitDirectory()
     {

src/GitVersionCore/GitPreparer.cs

@@ -160,9 +160,12 @@ public string GetProjectRootDirectory()
             }
 
             var dotGetGitDirectory = GetDotGitDirectory();
-            var result = Directory.GetParent(dotGetGitDirectory).FullName;
-            Logger.WriteInfo($"Returning Project Root from DotGitDirectory: {dotGetGitDirectory} - {result}");
-            return result;
+            using (var repo = new Repository(dotGetGitDirectory))
+            {
+                var result = repo.Info.WorkingDirectory;
+                Logger.WriteInfo($"Returning Project Root from DotGitDirectory: {dotGetGitDirectory} - {result}");
+                return result;
+            }
         }
 
         static string CreateDynamicRepository(string targetPath, AuthenticationInfo authentication, string repositoryUrl, string targetBranch, bool noFetch)