feat(config): add ability to set value of multivar variable

lib/src/bindings/config.dart

@@ -280,3 +280,23 @@ List<String> getMultivar(
 
   return entries;
 }
+
+/// Set the value of a multivar config variable in the config file with the
+/// highest level (usually the local one).
+///
+/// The regular expression is applied case-sensitively on the value.
+void setMultivar(
+  Pointer<git_config> cfg,
+  String name,
+  String regexp,
+  String value,
+) {
+  final nameC = name.toNativeUtf8().cast<Int8>();
+  final regexpC = regexp.toNativeUtf8().cast<Int8>();
+  final valueC = value.toNativeUtf8().cast<Int8>();
+  libgit2.git_config_set_multivar(cfg, nameC, regexpC, valueC);
+
+  calloc.free(nameC);
+  calloc.free(regexpC);
+  calloc.free(valueC);
+}

lib/src/config.dart

@@ -113,7 +113,7 @@ class Config {
   /// Map of key/value entries from config file.
   Map<String, dynamic> variables = {};
 
-  /// Sets value of config key
+  /// Sets the [value] of config [key]
   void setVariable(String key, dynamic value) {
     try {
       if (value.runtimeType == bool) {
@@ -150,6 +150,15 @@ class Config {
     return config.getMultivar(configPointer.value, key, regexp);
   }
 
+  /// Sets the [value] of a multivar [key] in the config file with the
+  /// highest level (usually the local one).
+  ///
+  /// The [regexp] is applied case-sensitively on the value.
+  /// Empty [regexp] sets [value] for all values of a multivar [key]
+  void setMultivar(String key, String regexp, String value) {
+    config.setMultivar(configPointer.value, key, regexp, value);
+  }
+
   /// Releases memory allocated for config object.
   void close() {
     calloc.free(configPointer);

test/config_test.dart

@@ -10,12 +10,12 @@ void main() {
   const configFileName = 'test_config';
   const contents = '''
 [core]
-  repositoryformatversion = 0
+\trepositoryformatversion = 0
-  bare = false
+\tbare = false
-  gitproxy = proxy-command for kernel.org
+\tgitproxy = proxy-command for kernel.org
-  gitproxy = default-proxy
+\tgitproxy = default-proxy
 [remote "origin"]
-  url = someurl
+\turl = someurl
 ''';
 
   late Config config;
@@ -90,6 +90,22 @@ void main() {
         ['proxy-command for kernel.org'],
       );
     });
+
+    test('sets value of multivar', () {
+      config.setMultivar('core.gitproxy', 'default', 'updated');
+      final multivarValues = config.getMultivar('core.gitproxy');
+      expect(multivarValues, isNot(contains('default-proxy')));
+      expect(multivarValues, contains('updated'));
+    });
+
+    test('sets value for all multivar values when regexp is empty', () {
+      config.setMultivar('core.gitproxy', '', 'updated');
+      final multivarValues = config.getMultivar('core.gitproxy');
+      expect(multivarValues, isNot(contains('default-proxy')));
+      expect(multivarValues, isNot(contains('proxy-command for kernel.org')));
+      expect(multivarValues, contains('updated'));
+      expect(multivarValues.length, equals(2));
+    });
   });
   ;
 }