fix: use messageonly type when notable-changes sec release

Partially fixes: nodejs-private/security-release#48

Author: RafaelGSS

lib/prepare_release.js

@@ -481,7 +481,10 @@ export default class ReleasePreparation extends Session {
     const data = await fs.readFile(majorChangelogPath, 'utf8');
     const arr = data.split('\n');
     const allCommits = this.getChangelog();
-    const notableChanges = await this.getBranchDiff({ onlyNotableChanges: true });
+    const notableChanges = await this.getBranchDiff({
+      onlyNotableChanges: true,
+      format: isSecurityRelease ? 'messageonly' : 'markdown',
+    });
     let releaseHeader = `## ${date}, Version ${newVersion}` +
                           ` ${releaseInfo}, @${username}\n`;
     if (isSecurityRelease) {