feat(index): add ability to read tree and write tree

lib/src/bindings/index.dart

@@ -25,6 +25,41 @@ void read(Pointer<git_index> index, bool force) {
   }
 }
 
+/// Read a tree into the index file with stats.
+///
+/// The current index contents will be replaced by the specified tree.
+///
+/// Throws a [LibGit2Error] if error occured.
+void readTree(Pointer<git_index> index, Pointer<git_tree> tree) {
+  final error = libgit2.git_index_read_tree(index, tree);
+
+  if (error < 0) {
+    throw LibGit2Error(libgit2.git_error_last());
+  }
+}
+
+/// Write the index as a tree.
+///
+/// This method will scan the index and write a representation of its current state back to disk;
+/// it recursively creates tree objects for each of the subtrees stored in the index, but only
+/// returns the OID of the root tree. This is the OID that can be used e.g. to create a commit.
+///
+/// The index instance cannot be bare, and needs to be associated to an existing repository.
+///
+/// The index must not contain any file in conflict.
+///
+/// Throws a [LibGit2Error] if error occured.
+Pointer<git_oid> writeTree(Pointer<git_index> index) {
+  final out = calloc<git_oid>();
+  final error = libgit2.git_index_write_tree(out, index);
+
+  if (error < 0) {
+    throw LibGit2Error(libgit2.git_error_last());
+  } else {
+    return out;
+  }
+}
+
 /// Find the first position of any entries which point to given path in the Git index.
 bool find(Pointer<git_index> index, String path) {
   final pathC = path.toNativeUtf8().cast<Int8>();
@@ -223,5 +258,9 @@ void removeAll(Pointer<git_index> index, List<String> pathspec) {
   }
 }
 
+/// Get the repository this index relates to.
+Pointer<git_repository> owner(Pointer<git_index> index) =>
+    libgit2.git_index_owner(index);
+
 /// Free an existing index object.
 void free(Pointer<git_index> index) => libgit2.git_index_free(index);

lib/src/bindings/tree.dart

@@ -0,0 +1,48 @@
+import 'dart:ffi';
+import 'package:ffi/ffi.dart';
+import '../error.dart';
+import 'libgit2_bindings.dart';
+import '../util.dart';
+
+/// Get the id of a tree.
+Pointer<git_oid> id(Pointer<git_tree> tree) => libgit2.git_tree_id(tree);
+
+/// Lookup a tree object from the repository.
+///
+/// Throws a [LibGit2Error] if error occured.
+Pointer<git_tree> lookup(Pointer<git_repository> repo, Pointer<git_oid> id) {
+  final out = calloc<Pointer<git_tree>>();
+  final error = libgit2.git_tree_lookup(out, repo, id);
+
+  if (error < 0) {
+    throw LibGit2Error(libgit2.git_error_last());
+  } else {
+    return out.value;
+  }
+}
+
+/// Lookup a tree object from the repository, given a prefix of its identifier (short id).
+///
+/// Throws a [LibGit2Error] if error occured.
+Pointer<git_tree> lookupPrefix(
+  Pointer<git_repository> repo,
+  Pointer<git_oid> id,
+  int len,
+) {
+  final out = calloc<Pointer<git_tree>>();
+  final error = libgit2.git_tree_lookup_prefix(out, repo, id, len);
+
+  if (error < 0) {
+    throw LibGit2Error(libgit2.git_error_last());
+  } else {
+    return out.value;
+  }
+}
+
+/// Close an open tree.
+///
+/// You can no longer use the git_tree pointer after this call.
+///
+/// IMPORTANT: You MUST call this method when you stop using a tree to release memory.
+/// Failure to do so will cause a memory leak.
+void free(Pointer<git_tree> tree) => libgit2.git_tree_free(tree);