feat(index): add binding and API method for git_index_new (#33)

lib/src/bindings/index.dart

@@ -5,6 +5,18 @@ import 'package:libgit2dart/src/bindings/libgit2_bindings.dart';
 import 'package:libgit2dart/src/error.dart';
 import 'package:libgit2dart/src/util.dart';
 
+/// Create an in-memory index object.
+///
+/// This index object cannot be read/written to the filesystem, but may be
+/// used to perform in-memory index operations.
+///
+/// The index must be freed once it's no longer in use.
+Pointer<git_index> newInMemory() {
+  final out = calloc<Pointer<git_index>>();
+  libgit2.git_index_new(out);
+  return out.value;
+}
+
 /// Read index capabilities flags.
 int capabilities(Pointer<git_index> index) => libgit2.git_index_caps(index);
 

lib/src/index.dart

@@ -14,6 +14,14 @@ class Index with IterableMixin<IndexEntry> {
   /// **IMPORTANT**: Should be freed to release allocated memory.
   const Index(this._indexPointer);
 
+  /// Creates an in-memory index object.
+  ///
+  /// This index object cannot be read/written to the filesystem, but may be
+  /// used to perform in-memory index operations.
+  ///
+  /// **IMPORTANT**: Should be freed to release allocated memory.
+  Index.newInMemory() : _indexPointer = bindings.newInMemory();
+
   final Pointer<git_index> _indexPointer;
 
   /// Pointer to memory address for allocated index object.

test/index_test.dart

@@ -29,6 +29,10 @@ void main() {
     const fileSha = 'e69de29bb2d1d6434b8b29ae775ad8c2e48c5391';
     const featureFileSha = '9c78c21d6680a7ffebc76f7ac68cacc11d8f48bc';
 
+    test('creates new in memory index object', () {
+      expect(Index.newInMemory(), isA<Index>());
+    });
+
     test('returns full path to the index file on disk', () {
       expect(index.path, p.join(repo.path, 'index'));
     });