feat(config): add ability to delete variable

lib/src/bindings/config.dart

@@ -233,11 +233,22 @@ Map<String, dynamic> getVariables(Pointer<git_config> cfg) {
   return entries;
 }
 
+/// Delete a config variable from the config file with the highest level
+/// (usually the local one).
+///
+/// Throws a [LibGit2Error] if error occured.
+void deleteVariable(Pointer<git_config> cfg, String name) {
+  final nameC = name.toNativeUtf8().cast<Int8>();
+  final error = libgit2.git_config_delete_entry(cfg, nameC);
+  calloc.free(nameC);
+
+  if (error < 0) {
+    throw LibGit2Error(libgit2.git_error_last());
+  }
+}
+
 /// Iterate over the values of multivar
 // TODO
 
 /// Multivars variables get/set
 // TODO
-
-/// Deletes a config variable
-// TODO

lib/src/config.dart

@@ -129,6 +129,19 @@ class Config {
     }
   }
 
+  /// Deletes variable from the config file with the highest level
+  /// (usually the local one).
+  ///
+  /// Throws a [LibGit2Error] if error occured.
+  void deleteVariable(String key) {
+    try {
+      config.deleteVariable(configPointer.value, key);
+      variables = config.getVariables(configPointer.value);
+    } catch (e) {
+      rethrow;
+    }
+  }
+
   /// Releases memory allocated for config object.
   void close() {
     calloc.free(configPointer);

test/config_test.dart

@@ -3,6 +3,7 @@ import 'dart:io';
 import 'package:test/test.dart';
 import 'package:libgit2dart/src/util.dart';
 import 'package:libgit2dart/src/config.dart';
+import 'package:libgit2dart/src/error.dart';
 
 void main() {
   final tmpDir = Directory.systemTemp.path;
@@ -51,6 +52,18 @@ void main() {
       config.setVariable('remote.origin.url', 'updated');
       expect(config.variables['remote.origin.url'], equals('updated'));
     });
+
+    test('deletes variable', () {
+      config.deleteVariable('core.bare');
+      expect(config.variables['core.bare'], isNull);
+    });
+
+    test('throws on deleting non existing variable', () {
+      expect(
+        () => config.deleteVariable('not.there'),
+        throwsA(isA<LibGit2Error>()),
+      );
+    });
   });
   ;
 }