Fix: nested extends with relative path (fixes #6358)

lib/config/config-file.js

@@ -382,7 +382,7 @@ function applyExtends(config, filePath, relativeTo) {
              * file as the reference point. Otherwise, use as-is.
              */
             parentPath = (!isAbsolutePath(parentPath) ?
-                path.join(path.dirname(filePath), parentPath) :
+                path.join(relativeTo || path.dirname(filePath), parentPath) :
                 parentPath
             );
         }

tests/fixtures/config-file/extends-chain-2/.eslintrc.json

@@ -0,0 +1,3 @@
+{
+    "extends": "a"
+}

tests/lib/config/config-file.js

@@ -642,6 +642,21 @@ describe("ConfigFile", function() {
             });
         });
 
+        it("should load information from `extends` chain with relative path.", function() {
+            var config = ConfigFile.load(getFixturePath("extends-chain-2/.eslintrc.json"));
+
+            assert.deepEqual(config, {
+                env: {},
+                extends: "a",
+                globals: {},
+                parserOptions: {},
+                rules: {
+                    a: 2,       // from node_modules/eslint-config-a/index.js
+                    relative: 2 // from node_modules/eslint-config-a/relative.js
+                }
+            });
+        });
+
         describe("Plugins", function() {
 
             it("should load information from a YML file and load plugins", function() {