feat(commit): add ability to amend commit

lib/src/bindings/commit.dart

@@ -113,6 +113,56 @@ Pointer<git_oid> create({
   }
 }
 
+/// Amend an existing commit by replacing only non-null values.
+///
+/// This creates a new commit that is exactly the same as the old commit, except that
+/// any non-null values will be updated. The new commit has the same parents as the old commit.
+///
+/// The [updateRef] value works as in the regular [create], updating the ref to point to
+/// the newly rewritten commit. If you want to amend a commit that is not currently
+/// the tip of the branch and then rewrite the following commits to reach a ref, pass
+/// this as null and update the rest of the commit chain and ref separately.
+///
+/// Throws a [LibGit2Error] if error occured.
+Pointer<git_oid> amend({
+  required Pointer<git_repository> repoPointer,
+  required Pointer<git_commit> commitPointer,
+  String? updateRef,
+  required Pointer<git_signature>? authorPointer,
+  required Pointer<git_signature>? committerPointer,
+  String? messageEncoding,
+  required String? message,
+  required Pointer<git_tree>? treePointer,
+}) {
+  final out = calloc<git_oid>();
+  final updateRefC = updateRef?.toNativeUtf8().cast<Int8>() ?? nullptr;
+  final messageEncodingC =
+      messageEncoding?.toNativeUtf8().cast<Int8>() ?? nullptr;
+  final messageC = message?.toNativeUtf8().cast<Int8>() ?? nullptr;
+
+  final error = libgit2.git_commit_amend(
+    out,
+    commitPointer,
+    updateRefC,
+    authorPointer ?? nullptr,
+    committerPointer ?? nullptr,
+    messageEncodingC,
+    messageC,
+    treePointer ?? nullptr,
+  );
+
+  calloc.free(updateRefC);
+  calloc.free(messageEncodingC);
+  calloc.free(messageC);
+
+  if (error < 0) {
+    calloc.free(out);
+    throw LibGit2Error(libgit2.git_error_last());
+  } else {
+    return out;
+  }
+}
+
 /// Get the encoding for the message of a commit, as a string representing a standard encoding name.
 ///
 /// The encoding may be NULL if the encoding header in the commit is missing;

lib/src/commit.dart

@@ -58,6 +58,39 @@ class Commit {
     ));
   }
 
+  /// Amends an existing commit by replacing only non-null values.
+  ///
+  /// This creates a new commit that is exactly the same as the old commit, except that
+  /// any non-null values will be updated. The new commit has the same parents as the old commit.
+  ///
+  /// The [updateRef] value works as in the regular [create], updating the ref to point to
+  /// the newly rewritten commit. If you want to amend a commit that is not currently
+  /// the tip of the branch and then rewrite the following commits to reach a ref, pass
+  /// this as null and update the rest of the commit chain and ref separately.
+  ///
+  /// Throws a [LibGit2Error] if error occured.
+  static Oid amend({
+    required Repository repo,
+    required Commit commit,
+    Signature? author,
+    Signature? committer,
+    Tree? tree,
+    String? updateRef,
+    String? message,
+    String? messageEncoding,
+  }) {
+    return Oid(bindings.amend(
+      repoPointer: repo.pointer,
+      commitPointer: commit.pointer,
+      authorPointer: author?.pointer,
+      committerPointer: committer?.pointer,
+      treePointer: tree?.pointer,
+      updateRef: updateRef,
+      message: message,
+      messageEncoding: messageEncoding,
+    ));
+  }
+
   /// Returns the encoding for the message of a commit, as a string
   /// representing a standard encoding name.
   String get messageEncoding => bindings.messageEncoding(_commitPointer);

lib/src/repository.dart

@@ -509,6 +509,59 @@ class Repository {
     );
   }
 
+  /// Amends an existing commit by replacing only non-null values.
+  ///
+  /// This creates a new commit that is exactly the same as the old commit, except that
+  /// any non-null values will be updated. The new commit has the same parents as the old commit.
+  ///
+  /// The [updateRef] value works as in the regular [create], updating the ref to point to
+  /// the newly rewritten commit. If you want to amend a commit that is not currently
+  /// the tip of the branch and then rewrite the following commits to reach a ref, pass
+  /// this as null and update the rest of the commit chain and ref separately.
+  ///
+  /// Throws a [LibGit2Error] if error occured.
+  Oid amendCommit({
+    required Commit commit,
+    Signature? author,
+    Signature? committer,
+    Tree? tree,
+    String? updateRef,
+    String? message,
+    String? messageEncoding,
+  }) {
+    return Commit.amend(
+      repo: this,
+      commit: commit,
+      author: author,
+      committer: committer,
+      tree: tree,
+      updateRef: updateRef,
+      message: message,
+      messageEncoding: messageEncoding,
+    );
+  }
+
+  /// Reverts the given commit against the given "our" commit, producing an index that
+  /// reflects the result of the revert.
+  ///
+  /// [mainline] is parent of the [revertCommit] if it is a merge (i.e. 1, 2).
+  ///
+  /// The returned index must be freed explicitly with `free()`.
+  ///
+  /// Throws a [LibGit2Error] if error occured.
+  Index revertCommit({
+    required Commit revertCommit,
+    required Commit ourCommit,
+    mainline = 0,
+  }) {
+    return Index(commit_bindings.revertCommit(
+      repoPointer: _repoPointer,
+      revertCommitPointer: revertCommit.pointer,
+      ourCommitPointer: ourCommit.pointer,
+      mainline: mainline,
+    ));
+  }
+
   /// Finds a single object and intermediate reference (if there is one) by a [spec] revision string.
   ///
   /// See `man gitrevisions`, or https://git-scm.com/docs/git-rev-parse.html#_specifying_revisions
@@ -875,27 +928,6 @@ class Repository {
     ));
   }
 
-  /// Reverts the given commit against the given "our" commit, producing an index that
-  /// reflects the result of the revert.
-  ///
-  /// [mainline] is parent of the [revertCommit] if it is a merge (i.e. 1, 2).
-  ///
-  /// The returned index must be freed explicitly with `free()`.
-  ///
-  /// Throws a [LibGit2Error] if error occured.
-  Index revertCommit({
-    required Commit revertCommit,
-    required Commit ourCommit,
-    mainline = 0,
-  }) {
-    return Index(commit_bindings.revertCommit(
-      repoPointer: _repoPointer,
-      revertCommitPointer: revertCommit.pointer,
-      ourCommitPointer: ourCommit.pointer,
-      mainline: mainline,
-    ));
-  }
-
   /// Merges two trees, 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 part