refactor: use git_diff_to_buf to produce patch diff text

lib/src/bindings/diff.dart

@@ -365,14 +365,15 @@ String statsPrint({
   }
 }
 
-/// Add patch to buffer.
-Pointer<git_buf> addToBuf({
-  required Pointer<git_patch> patchPointer,
-  required Pointer<git_buf> bufferPointer,
-}) {
-  libgit2.git_patch_to_buf(bufferPointer, patchPointer);
-
-  return bufferPointer;
+/// Produce the complete formatted text output from a diff into a buffer.
+String addToBuf(Pointer<git_diff> diff) {
+  final out = calloc<git_buf>();
+  libgit2.git_diff_to_buf(out, diff, git_diff_format_t.GIT_DIFF_FORMAT_PATCH);
+  final result = out.ref.ptr == nullptr
+      ? ''
+      : out.ref.ptr.cast<Utf8>().toDartString(length: out.ref.size);
+  calloc.free(out);
+  return result;
 }
 
 /// Counter for hunk number being applied.

lib/src/diff.dart

@@ -293,26 +293,8 @@ class Diff {
     return patches;
   }
 
-  /// The patch diff string.
-  String get patch {
-    final length = bindings.length(_diffPointer);
-    var buffer = calloc<git_buf>(sizeOf<git_buf>());
-
-    for (var i = 0; i < length; i++) {
-      final patch = Patch.fromDiff(diff: this, index: i);
-      buffer = bindings.addToBuf(
-        patchPointer: patch.pointer,
-        bufferPointer: buffer,
-      );
-      patch.free();
-    }
-
-    final result = buffer.ref.ptr == nullptr
-        ? ''
-        : buffer.ref.ptr.cast<Utf8>().toDartString();
-    calloc.free(buffer);
-    return result;
-  }
+  /// The patch diff text.
+  String get patch => bindings.addToBuf(_diffPointer);
 
   /// Accumulates diff statistics for all patches.
   ///

lib/src/patch.dart

@@ -153,9 +153,6 @@ class Patch {
 
   late final Pointer<git_patch> _patchPointer;
 
-  /// Pointer to memory address for allocated patch object.
-  Pointer<git_patch> get pointer => _patchPointer;
-
   /// Line counts of each type in a patch.
   PatchStats get stats {
     final result = bindings.lineStats(_patchPointer);