create func to get revision body

src/server/models/page.js

@@ -1383,6 +1383,20 @@ module.exports = function(crowi) {
     return addSlashOfEnd(path);
   };
 
+  /**
+   * return revision body by revisionId
+   */
+  pageSchema.statics.getMarkdown = async function(revisionId) {
+    const Revision = crowi.model('Revision');
+    let markdown;
+
+    await Revision.findById(revisionId, (err, revision) => {
+      markdown = revision.body;
+    });
+
+    return markdown;
+  };
+
   pageSchema.statics.GRANT_PUBLIC = GRANT_PUBLIC;
   pageSchema.statics.GRANT_RESTRICTED = GRANT_RESTRICTED;
   pageSchema.statics.GRANT_SPECIFIED = GRANT_SPECIFIED;