feat(merge): add ability to merge file from index

lib/src/bindings/merge.dart

@@ -89,6 +89,36 @@ void merge({
   }
 }
 
+/// Merge two files as they exist in the index, using the given common ancestor
+/// as the baseline, producing a string that reflects the merge result containing
+/// possible conflicts.
+///
+/// Throws a [LibGit2Error] if error occured.
+String mergeFileFromIndex({
+  required Pointer<git_repository> repoPointer,
+  required Pointer<git_index_entry>? ancestorPointer,
+  required Pointer<git_index_entry>? oursPointer,
+  required Pointer<git_index_entry>? theirsPointer,
+}) {
+  final out = calloc<git_merge_file_result>();
+  final error = libgit2.git_merge_file_from_index(
+    out,
+    repoPointer,
+    ancestorPointer ?? nullptr,
+    oursPointer ?? nullptr,
+    theirsPointer ?? nullptr,
+    nullptr,
+  );
+
+  if (error < 0) {
+    throw LibGit2Error(libgit2.git_error_last());
+  } else {
+    final result = out.ref.ptr.cast<Utf8>().toDartString(length: out.ref.len);
+    calloc.free(out);
+    return result;
+  }
+}
+
 /// Merge two commits, producing a git_index that reflects the result of the merge.
 /// The index may be written as-is to the working directory or checked out. If the index
 /// is to be converted to a tree, the caller should resolve any conflicts that arose as

lib/src/index.dart

@@ -256,9 +256,11 @@ class IndexEntry {
   /// Initializes a new instance of [IndexEntry] class.
   const IndexEntry(this._indexEntryPointer);
 
-  /// Pointer to memory address for allocated index entry object.
   final Pointer<git_index_entry> _indexEntryPointer;
 
+  /// Pointer to memory address for allocated index entry object.
+  Pointer<git_index_entry> get pointer => _indexEntryPointer;
+
   /// Unique identity of the index entry.
   Oid get id => Oid.fromRaw(_indexEntryPointer.ref.id);
 
@@ -282,6 +284,11 @@ class IndexEntry {
   /// Sets the UNIX file attributes of a index entry.
   set mode(GitFilemode mode) => _indexEntryPointer.ref.mode = mode.value;
 
+  @override
+  String toString() {
+    return 'IndexEntry{path: $path, sha: $sha}';
+  }
+
   String _oidToHex(git_oid oid) {
     var hex = StringBuffer();
     for (var i = 0; i < 20; i++) {

lib/src/repository.dart

@@ -677,6 +677,24 @@ class Repository {
     commit_bindings.annotatedFree(theirHead.value);
   }
 
+  /// Merges two files as they exist in the index, using the given common ancestor
+  /// as the baseline, producing a string that reflects the merge result containing
+  /// possible conflicts.
+  ///
+  /// Throws a [LibGit2Error] if error occured.
+  String mergeFileFromIndex({
+    required IndexEntry? ancestor,
+    required IndexEntry? ours,
+    required IndexEntry? theirs,
+  }) {
+    return merge_bindings.mergeFileFromIndex(
+      repoPointer: _repoPointer,
+      ancestorPointer: ancestor?.pointer,
+      oursPointer: ours?.pointer,
+      theirsPointer: theirs?.pointer,
+    );
+  }
+
   /// Merges two commits, producing an index that reflects the result of the merge.
   /// The index may be written as-is to the working directory or checked out. If the index
   /// is to be converted to a tree, the caller should resolve any conflicts that arose as