feat(graph): add bindings and api

lib/src/bindings/graph.dart

@@ -0,0 +1,53 @@
+import 'dart:ffi';
+import 'package:ffi/ffi.dart';
+import 'libgit2_bindings.dart';
+import '../error.dart';
+import '../util.dart';
+
+/// Determine if a commit is the descendant of another commit.
+///
+/// Note that a commit is not considered a descendant of itself, in contrast to
+/// `git merge-base --is-ancestor`.
+///
+/// Throws a [LibGit2Error] if error occured.
+bool descendantOf({
+  required Pointer<git_repository> repoPointer,
+  required Pointer<git_oid> commitPointer,
+  required Pointer<git_oid> ancestorPointer,
+}) {
+  final result = libgit2.git_graph_descendant_of(
+    repoPointer,
+    commitPointer,
+    ancestorPointer,
+  );
+
+  if (result < 0) {
+    throw LibGit2Error(libgit2.git_error_last());
+  } else {
+    return result == 1 ? true : false;
+  }
+}
+
+/// Count the number of unique commits between two commit objects.
+///
+/// There is no need for branches containing the commits to have any upstream relationship,
+/// but it helps to think of one as a branch and the other as its upstream, the ahead and
+/// behind values will be what git would report for the branches.
+List<int> aheadBehind({
+  required Pointer<git_repository> repoPointer,
+  required Pointer<git_oid> localPointer,
+  required Pointer<git_oid> upstreamPointer,
+}) {
+  final ahead = calloc<Uint64>();
+  final behind = calloc<Uint64>();
+
+  libgit2.git_graph_ahead_behind(
+    ahead,
+    behind,
+    repoPointer,
+    localPointer,
+    upstreamPointer,
+  );
+
+  return [ahead.value, behind.value];
+}