build: configure dev-infra merge script (#37138)

Sets up the dev-infa merge script in the framework ng-dev configuration
file. This allow us to use the script in the future.

PR Close #37138

.ng-dev-config.ts

@@ -1,3 +1,7 @@
+import {exec} from 'shelljs';
+
+import {MergeConfig} from './dev-infra/pr/merge/config';
+
 // The configuration for `ng-dev commit-message` commands.
 const commitMessage = {
   'maxLength': 120,
@@ -72,15 +76,72 @@ const format = {
   'buildifier': true
 };
 
-// Github metadata information for `ng-dev` commands.
+/** Github metadata information for `ng-dev` commands. */
 const github = {
   owner: 'angular',
   name: 'angular',
 };
 
+/**
+ * Gets the name of the current patch branch. The patch branch is determined by
+ * looking for upstream branches that follow the format of `{major}.{minor}.x`.
+ */
+const getPatchBranchName = (): string => {
+  const branches =
+      exec(
+          `git ls-remote --heads https://github.com/${github.owner}/${github.name}.git`,
+          {silent: true})
+          .trim()
+          .split('\n');
+
+  for (let i = branches.length - 1; i >= 0; i--) {
+    const branchName = branches[i];
+    const matches = branchName.match(/refs\/heads\/([0-9]+\.[0-9]+\.x)/);
+    if (matches !== null) {
+      return matches[1];
+    }
+  }
+
+  throw Error('Could not determine patch branch name.');
+};
+
+// Configuration for the `ng-dev pr merge` command. The command can be used
+// for merging upstream pull requests into branches based on a PR target label.
+const merge = () => {
+  const patchBranch = getPatchBranchName();
+  const config: MergeConfig = {
+    githubApiMerge: false,
+    claSignedLabel: 'cla: yes',
+    mergeReadyLabel: /^PR action: merge(-assistance)?/,
+    commitMessageFixupLabel: 'commit message fixup',
+    labels: [
+      {
+        pattern: 'PR target: master-only',
+        branches: ['master'],
+      },
+      {
+        pattern: 'PR target: patch-only',
+        branches: [patchBranch],
+      },
+      {
+        pattern: 'PR target: master & patch',
+        branches: ['master', patchBranch],
+      },
+    ],
+    requiredBaseCommits: {
+      // PRs that target either `master` or the patch branch, need to be rebased
+      // on top of the latest commit message validation fix.
+      'master': '4341743b4a6d7e23c6f944aa9e34166b701369a1',
+      [patchBranch]: '2a53f471592f424538802907aca1f60f1177a86d'
+    },
+  };
+  return config;
+};
+
 // Export function to build ng-dev configuration object.
 module.exports = {
   commitMessage,
   format,
   github,
+  merge,
 };