Implement git clean

This implements git clean and exposes the ability to remove untracked files from the working directory.
If / when git clean functionality is exposed directly by libgit2, we can update this to use that entry point.
Currently, we call git_checkout_index with a flags indicating that untracked entries should be removed. This does
not support other flags exposed by core Git, such as removing ignored files.

Author: jamill

LibGit2Sharp.Tests/IndexFixture.cs

@@ -82,6 +82,24 @@ public void ReadIndexWithBadParamsFails()
             }
         }
 
+        [Fact]
+        public void CanCleanWorkingDirectory()
+        {
+            TemporaryCloneOfTestRepo path = BuildTemporaryCloneOfTestRepo(StandardTestRepoWorkingDirPath);
+            using (var repo = new Repository(path.RepositoryPath))
+            {
+                // Verify that there are the expected number of entries and untracked files
+                Assert.Equal(6, repo.Index.RetrieveStatus().Count());
+                Assert.Equal(1, repo.Index.RetrieveStatus().Untracked.Count());
+
+                repo.Index.CleanWorkingDirectory();
+
+                // Verify that there are the expected number of entries and 0 untracked files
+                Assert.Equal(5, repo.Index.RetrieveStatus().Count());
+                Assert.Equal(0, repo.Index.RetrieveStatus().Untracked.Count());
+            }
+        }
+
         [Theory]
         [InlineData("1/branch_file.txt", FileStatus.Unaltered, true, FileStatus.Unaltered, true, 0)]
         [InlineData("README", FileStatus.Unaltered, true, FileStatus.Unaltered, true, 0)]

LibGit2Sharp/Core/NativeMethods.cs

@@ -157,6 +157,11 @@ internal static extern int git_checkout_head(
             RepositorySafeHandle repo,
             GitCheckoutOpts opts);
 
+        [DllImport(libgit2)]
+        internal static extern int git_checkout_index(
+            RepositorySafeHandle repo,
+            GitCheckoutOpts opts);
+
         [DllImport(libgit2)]
         internal static extern int git_clone(
             out RepositorySafeHandle repo,

LibGit2Sharp/Core/Proxy.cs

@@ -188,6 +188,15 @@ public static void git_checkout_head(RepositorySafeHandle repo, GitCheckoutOpts
             }
         }
 
+        public static void git_checkout_index(RepositorySafeHandle repo, GitCheckoutOpts opts)
+        {
+            using (ThreadAffinity())
+            {
+                int res = NativeMethods.git_checkout_index(repo, opts);
+                Ensure.Success(res);
+            }
+        }
+
         #endregion
 
         #region git_clone_

LibGit2Sharp/Index.cs

@@ -338,6 +338,19 @@ public virtual void Remove(IEnumerable<string> paths)
             UpdatePhysicalIndex();
         }
 
+        /// <summary>
+        /// Clean the working tree by removing files that are not under version control.
+        /// </summary>
+        public virtual void CleanWorkingDirectory()
+        {
+            GitCheckoutOpts options = new GitCheckoutOpts
+            {
+                checkout_strategy = CheckoutStrategy.GIT_CHECKOUT_REMOVE_UNTRACKED,
+            };
+
+            Proxy.git_checkout_index(this.repo.Handle, options);
+        }
+
         private IEnumerable<KeyValuePair<string, FileStatus>> PrepareBatch(IEnumerable<string> paths)
         {
             Ensure.ArgumentNotNull(paths, "paths");