fix($core): use directory name to compute slug if filename is readme …

…or index (close: #1443) (#1535)

packages/@vuepress/core/lib/node/Page.js

@@ -165,7 +165,19 @@ module.exports = class Page {
    */
 
   get slug () {
-    return slugify(this.strippedFilename)
+    const strippedFilename = this.strippedFilename
+
+    if (/^(index|readme)$/i.test(strippedFilename)) {
+      const strippedFilename = this.stripFilename(
+        path.basename(path.dirname(this._filePath || this.regularPath))
+      )
+
+      if (strippedFilename) {
+        return slugify(strippedFilename)
+      }
+    }
+
+    return slugify(strippedFilename)
   }
 
   /**
@@ -179,8 +191,7 @@ module.exports = class Page {
    */
 
   get strippedFilename () {
-    const match = this.filename.match(DATE_RE)
-    return match ? match[3] : this.filename
+    return this.stripFilename(this.filename)
   }
 
   /**
@@ -194,6 +205,22 @@ module.exports = class Page {
     return inferDate(this.frontmatter, this.filename)
   }
 
+  /**
+   * stripped file name.
+   *
+   * If filename was yyyy-MM-dd-[title], the date prefix will be stripped.
+   * If filename was yyyy-MM-[title], the date prefix will be stripped.
+   *
+   * @param {string} fileName
+   * @returns {string}
+   * @private
+   */
+  stripFilename(fileName) {
+    const match = fileName.match(DATE_RE)
+
+    return match ? match[3] : fileName
+  }
+
   /**
    * Convert page's metadata to JSON, note that all fields beginning
    * with an underscore will not be serialized.