feat(signature): add bindings and api for default signature

lib/src/bindings/signature.dart

@@ -50,5 +50,22 @@ Pointer<git_signature> now(String name, String email) {
   }
 }
 
+/// Create a new action signature with default user and now timestamp.
+///
+/// This looks up the user.name and user.email from the configuration and uses the
+/// current time as the timestamp, and creates a new signature based on that information.
+///
+/// Throws a [LibGit2Error] if error occured.
+Pointer<git_signature> defaultSignature(Pointer<git_repository> repo) {
+  final out = calloc<Pointer<git_signature>>();
+  final error = libgit2.git_signature_default(out, repo);
+
+  if (error < 0) {
+    throw LibGit2Error(libgit2.git_error_last());
+  } else {
+    return out.value;
+  }
+}
+
 /// Free an existing signature.
 void free(Pointer<git_signature> sig) => libgit2.git_signature_free(sig);

lib/src/repository.dart

@@ -361,6 +361,14 @@ class Repository {
     }
   }
 
+  /// Creates a new action signature with default user and now timestamp.
+  ///
+  /// This looks up the user.name and user.email from the configuration and uses the
+  /// current time as the timestamp, and creates a new signature based on that information.
+  ///
+  /// Throws a [LibGit2Error] if error occured.
+  Signature get defaultSignature => Signature.defaultSignature(this);
+
   /// Returns the list of commits starting from provided [sha] hex string.
   ///
   /// If [sorting] isn't provided default will be used (reverse chronological order, like in git).

lib/src/signature.dart

@@ -2,6 +2,7 @@ import 'dart:ffi';
 import 'package:ffi/ffi.dart';
 import 'bindings/libgit2_bindings.dart';
 import 'bindings/signature.dart' as bindings;
+import 'repository.dart';
 import 'util.dart';
 
 class Signature {
@@ -39,6 +40,15 @@ class Signature {
   /// Pointer to memory address for allocated signature object.
   Pointer<git_signature> get pointer => _signaturePointer;
 
+  /// Creates a new action signature with default user and now timestamp.
+  ///
+  /// This looks up the user.name and user.email from the configuration and uses the
+  /// current time as the timestamp, and creates a new signature based on that information.
+  ///
+  /// Throws a [LibGit2Error] if error occured.
+  static Signature defaultSignature(Repository repo) =>
+      Signature(bindings.defaultSignature(repo.pointer));
+
   /// Returns full name of the author.
   String get name => _signaturePointer.ref.name.cast<Utf8>().toDartString();