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/CleanFixture.cs

@@ -0,0 +1,36 @@
+using System.Linq;
+using LibGit2Sharp.Tests.TestHelpers;
+using Xunit;
+
+namespace LibGit2Sharp.Tests
+{
+    public class CleanFixture : BaseFixture
+    {
+        [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.RemoveUntrackedFiles();
+
+                // 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());
+            }
+        }
+
+        [Fact]
+        public void CannotCleanABareRepository()
+        {
+            using (var repo = new Repository(BareTestRepoPath))
+            {
+                Assert.Throws<BareRepositoryException>(() => repo.RemoveUntrackedFiles());
+            }
+        }
+    }
+}

LibGit2Sharp.Tests/LibGit2Sharp.Tests.csproj

@@ -58,6 +58,7 @@
   </ItemGroup>
   <ItemGroup>
     <Compile Include="CheckoutFixture.cs" />
+    <Compile Include="CleanFixture.cs" />
     <Compile Include="MetaFixture.cs" />
     <Compile Include="MockedRepositoryFixture.cs" />
     <Compile Include="ConfigurationFixture.cs" />

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/Repository.cs

@@ -616,6 +616,18 @@ private IEnumerable<Commit> RetrieveParentsOfTheCommitBeingCreated(bool amendPre
             return new[] { Head.Tip };
         }
 
+        /// <summary>
+        /// Clean the working tree by removing files that are not under version control.
+        /// </summary>
+        public virtual void RemoveUntrackedFiles()
+        {
+            var options = new GitCheckoutOpts
+            {
+                checkout_strategy = CheckoutStrategy.GIT_CHECKOUT_REMOVE_UNTRACKED,
+            };
+
+            Proxy.git_checkout_index(Handle, options);
+        }
 
         internal T RegisterForCleanup<T>(T disposable) where T : IDisposable
         {