build: modularize the gulp file to be easier to maintain (#14259)

This is a precursor to bringing in some of the gulp tasks from angular.io,
which could lead to the gulpfile becoming unwieldy.

tools/gulp-tasks/validate-commit-message.js

@@ -0,0 +1,33 @@
+module.exports = (gulp) => () => {
+  const validateCommitMessage = require('../validate-commit-message');
+  const childProcess = require('child_process');
+
+  // We need to fetch origin explicitly because it might be stale.
+  // I couldn't find a reliable way to do this without fetch.
+  childProcess.exec(
+      'git fetch origin master && git log --reverse --format=%s HEAD ^origin/master',
+      (error, stdout, stderr) => {
+        if (error) {
+          console.log(stderr);
+          process.exit(1);
+        }
+
+        let someCommitsInvalid = false;
+        let commitsByLine = stdout.trim().split(/\n/);
+
+        console.log(`Examining ${commitsByLine.length} commits between HEAD and master`);
+
+        if (commitsByLine.length == 0) {
+          console.log('There are zero new commits between this HEAD and master');
+        }
+
+        someCommitsInvalid = !commitsByLine.every(validateCommitMessage);
+
+        if (someCommitsInvalid) {
+          console.log('Please fix the failing commit messages before continuing...');
+          console.log(
+              'Commit message guidelines: https://github.com/angular/angular/blob/master/CONTRIBUTING.md#-commit-message-guidelines');
+          process.exit(1);
+        }
+      });
+};