now compares against the last run branch built on

package.json

@@ -56,6 +56,7 @@
     "fbjs-scripts": "^0.6.0",
     "filesize": "^3.5.6",
     "flow-bin": "^0.41.0",
+    "git-branch": "^0.3.0",
     "glob": "^6.0.4",
     "glob-stream": "^6.1.0",
     "gzip-js": "~0.3.2",

scripts/rollup/build.js

@@ -43,6 +43,7 @@ const {
  } = require('./bundles');
 const { propertyMangleWhitelist } = require('./mangle');
 const sizes = require('./sizes-plugin');
+const branch = require('git-branch');
 
 const errorCodeOpts = {
   errorMapFilePath: 'scripts/error-codes/codes.json',
@@ -388,7 +389,9 @@ function createNodePackage(bundleType, packageName, filename) {
 }
 
 function saveResults() {
-  const contents = {};
+  const contents = {
+    branch: branch.sync(),
+  };
 
   results.forEach(({ filename, size, gzip }) => {
     contents[filename] = {
@@ -404,7 +407,7 @@ function percentChange(prev, current) {
   const change = Math.floor((current - prev) / prev * 100);
 
   if (change > 0) {
-    return chalk.red.bold(change + ' %');
+    return chalk.red.bold(`+${change} %`);
   } else if (change <= 0) {
     return chalk.green.bold(change + ' %');
   }
@@ -437,7 +440,10 @@ function printResults() {
         percentChange(prevGzip, gzip),
     ]);
   });
-  return table.toString();
+  return (
+    table.toString() + 
+    `\n\nThe difference was compared to the last build on "${ chalk.green.bold(prevResults.branch) }" branch.\n`
+  );
 }
 
 function getPlugins(entry, babelOpts, paths, filename, bundleType, isRenderer, manglePropertiesOnProd) {

scripts/rollup/results.json

@@ -1 +1 @@
-{"React-prod.js":{"filename":"React-prod.js","gzip":14394,"size":56438},"ReactDOMStack-prod.js":{"filename":"ReactDOMStack-prod.js","gzip":85702,"size":355558},"ReactDOMFiber-prod.js":{"filename":"ReactDOMFiber-prod.js","gzip":94702,"size":412153},"ReactDOMServerStack-prod.js":{"filename":"ReactDOMServerStack-prod.js","gzip":81496,"size":336791},"ReactARTStack-prod.js":{"filename":"ReactARTStack-prod.js","gzip":88945,"size":368987},"ReactARTFiber-prod.js":{"filename":"ReactARTFiber-prod.js","gzip":43477,"size":207824}}
+{"branch":"rollup","ReactNative.js":{"filename":"ReactNative.js","gzip":555,"size":1569}}