fix: use plaintext formatting in release commit (nodejs#417)

Author: codebytere

lib/prepare_release.js

@@ -377,7 +377,7 @@ class ReleasePreparation {
 
     const notableChanges = this.getBranchDiff({
       onlyNotableChanges: true,
-      simple: true
+      format: 'plaintext'
     });
     messageBody.push('Notable changes:\n\n');
     messageBody.push(notableChanges);
@@ -431,12 +431,9 @@ class ReleasePreparation {
       branchDiffOptions = [
         `${upstream}/${releaseBranch}`,
         proposalBranch,
-        `--require-label=${notableLabels.join(',')}`
+        `--require-label=${notableLabels.join(',')}`,
+        `--format=${opts.format || 'markdown'}`
       ];
-
-      if (opts.simple) {
-        branchDiffOptions.push('--simple');
-      }
     } else {
       const excludeLabels = [
         'semver-major',