Expose the prune option for fetching

LibGit2Sharp.Tests/NetworkFixture.cs

@@ -263,5 +263,40 @@ public void CanMergeFetchedRefs()
                 Assert.Equal(mergeResult.Status, MergeStatus.NonFastForward);
             }
         }
+
+        [Fact]
+        public void CanPruneRefs()
+        {
+            string url = "https://github.com/libgit2/TestGitRepository";
+
+            var scd = BuildSelfCleaningDirectory();
+            string clonedRepoPath = Repository.Clone(url, scd.DirectoryPath);
+
+            var scd2 = BuildSelfCleaningDirectory();
+            string clonedRepoPath2 = Repository.Clone(url, scd2.DirectoryPath);
+
+
+            using (var repo = new Repository(clonedRepoPath))
+            {
+                repo.Network.Remotes.Add("pruner", "file://" + clonedRepoPath2);
+                var remote = repo.Network.Remotes["pruner"];
+                repo.Network.Fetch(remote);
+                Assert.NotNull(repo.Refs["refs/remotes/pruner/master"]);
+
+                // Remove the branch from the source repository
+                using (var repo2 = new Repository(clonedRepoPath2))
+                {
+                    repo2.Refs.Remove("refs/heads/master");
+                }
+
+                // and by default we don't prune it
+                repo.Network.Fetch(remote);
+                Assert.NotNull(repo.Refs["refs/remotes/pruner/master"]);
+
+                // but we do when asked by the user
+                repo.Network.Fetch(remote, new FetchOptions { Prune = true} );
+                Assert.Null(repo.Refs["refs/remotes/pruner/master"]);
+            }
+        }
     }
 }

LibGit2Sharp/FetchOptions.cs

@@ -16,5 +16,14 @@ public sealed class FetchOptions : FetchOptionsBase
         /// retrieved during this fetch will be retrieved as well).</para>
         /// </summary>
         public TagFetchMode? TagFetchMode { get; set; }
+
+        /// <summary>
+        /// Specifies the pruning behaviour for the fetch operation
+        /// <para>
+        /// If not set, the configuration's setting will take effect. If true, the branches which no longer
+        /// exist on the remote will be removed from the remote-tracking branches.
+        /// </para>
+        /// </summary>
+        public bool? Prune { get; set; }
     }
 }

LibGit2Sharp/Network.cs

@@ -206,6 +206,15 @@ private static void DoFetch(FetchOptions options, RemoteSafeHandle remoteHandle,
                 fetchOptions.download_tags = options.TagFetchMode.Value;
             }
 
+            if (options.Prune.HasValue)
+            {
+                fetchOptions.Prune = options.Prune.Value ? FetchPruneStrategy.Prune : FetchPruneStrategy.NoPrune;
+            }
+            else
+            {
+                fetchOptions.Prune = FetchPruneStrategy.FromConfigurationOrDefault;
+            }
+
             Proxy.git_remote_fetch(remoteHandle, refspecs, fetchOptions, logMessage);
         }