feat(merge): add more bindings and api methods

lib/src/bindings/merge.dart

@@ -54,3 +54,115 @@ List<int> analysis(
     return result;
   }
 }
+
+/// Merges the given commit(s) into HEAD, writing the results into the working directory.
+/// Any changes are staged for commit and any conflicts are written to the index. Callers
+/// should inspect the repository's index after this completes, resolve any conflicts and
+/// prepare a commit.
+///
+/// Throws a [LibGit2Error] if error occured.
+void merge(
+  Pointer<git_repository> repo,
+  Pointer<Pointer<git_annotated_commit>> theirHeads,
+  int theirHeadsLen,
+) {
+  final mergeOpts = calloc<git_merge_options>(sizeOf<git_merge_options>());
+  libgit2.git_merge_options_init(mergeOpts, 1);
+
+  final checkoutOpts =
+      calloc<git_checkout_options>(sizeOf<git_checkout_options>());
+  libgit2.git_checkout_options_init(checkoutOpts, 1);
+  checkoutOpts.ref.checkout_strategy =
+      git_checkout_strategy_t.GIT_CHECKOUT_SAFE +
+          git_checkout_strategy_t.GIT_CHECKOUT_RECREATE_MISSING;
+
+  final error = libgit2.git_merge(
+    repo,
+    theirHeads,
+    theirHeadsLen,
+    mergeOpts,
+    checkoutOpts,
+  );
+
+  if (error < 0) {
+    throw LibGit2Error(libgit2.git_error_last());
+  }
+}
+
+/// 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
+/// part of the merge.
+///
+/// The returned index must be freed explicitly.
+///
+/// Throws a [LibGit2Error] if error occured.
+Pointer<git_index> mergeCommits(
+  Pointer<git_repository> repo,
+  Pointer<git_commit> ourCommit,
+  Pointer<git_commit> theirCommit,
+  Map<String, int> opts,
+) {
+  final out = calloc<Pointer<git_index>>();
+  final optsC = calloc<git_merge_options>(sizeOf<git_merge_options>());
+  optsC.ref.file_favor = opts['favor']!;
+  optsC.ref.flags = opts['mergeFlags']!;
+  optsC.ref.file_flags = opts['fileFlags']!;
+  optsC.ref.version = GIT_MERGE_OPTIONS_VERSION;
+
+  final error = libgit2.git_merge_commits(
+    out,
+    repo,
+    ourCommit,
+    theirCommit,
+    optsC,
+  );
+
+  calloc.free(optsC);
+
+  if (error < 0) {
+    throw LibGit2Error(libgit2.git_error_last());
+  } else {
+    return out.value;
+  }
+}
+
+/// Merge two trees, 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 part
+/// of the merge.
+///
+/// The returned index must be freed explicitly.
+///
+/// Throws a [LibGit2Error] if error occured.
+Pointer<git_index> mergeTrees(
+  Pointer<git_repository> repo,
+  Pointer<git_tree> ancestorTree,
+  Pointer<git_tree> ourTree,
+  Pointer<git_tree> theirTree,
+  Map<String, int> opts,
+) {
+  final out = calloc<Pointer<git_index>>();
+  final optsC = calloc<git_merge_options>(sizeOf<git_merge_options>());
+  optsC.ref.file_favor = opts['favor']!;
+  optsC.ref.flags = opts['mergeFlags']!;
+  optsC.ref.file_flags = opts['fileFlags']!;
+  optsC.ref.version = GIT_MERGE_OPTIONS_VERSION;
+
+  final error = libgit2.git_merge_trees(
+    out,
+    repo,
+    ancestorTree,
+    ourTree,
+    theirTree,
+    optsC,
+  );
+
+  calloc.free(optsC);
+
+  if (error < 0) {
+    throw LibGit2Error(libgit2.git_error_last());
+  } else {
+    return out.value;
+  }
+}