feat(dev-infra): tooling to check out pending PR (#38474)

Creates a tool within ng-dev to checkout a pending PR from the upstream repository.  This automates
an action that many developers on the Angular team need to do periodically in the process of testing
and reviewing incoming PRs.

Example usage:
  ng-dev pr checkout <pr-number>

PR Close #38474

dev-infra/utils/git/github.ts

@@ -26,7 +26,7 @@ export class GithubApiRequestError extends Error {
  **/
 export class GithubClient extends Octokit {
   /** The Github GraphQL (v4) API. */
-  graqhql: GithubGraphqlClient;
+  graphql: GithubGraphqlClient;
 
   /** The current user based on checking against the Github API. */
   private _currentUser: string|null = null;
@@ -42,7 +42,7 @@ export class GithubClient extends Octokit {
     });
 
     // Create authenticated graphql client.
-    this.graqhql = new GithubGraphqlClient(token);
+    this.graphql = new GithubGraphqlClient(token);
   }
 
   /** Retrieve the login of the current user from Github. */
@@ -51,7 +51,7 @@ export class GithubClient extends Octokit {
     if (this._currentUser !== null) {
       return this._currentUser;
     }
-    const result = await this.graqhql.query({
+    const result = await this.graphql.query({
       viewer: {
         login: types.string,
       }
@@ -80,7 +80,7 @@ class GithubGraphqlClient {
     // Set the default headers to include authorization with the provided token for all
     // graphQL calls.
     if (token) {
-      this.graqhql.defaults({headers: {authorization: `token ${token}`}});
+      this.graqhql = this.graqhql.defaults({headers: {authorization: `token ${token}`}});
     }
   }
 

dev-infra/utils/git/index.ts

@@ -150,6 +150,25 @@ export class GitClient {
     return value.replace(this._githubTokenRegex, '<TOKEN>');
   }
 
+  /**
+   * Checks out a requested branch or revision, optionally cleaning the state of the repository
+   * before attempting the checking. Returns a boolean indicating whether the branch or revision
+   * was cleanly checked out.
+   */
+  checkout(branchOrRevision: string, cleanState: boolean): boolean {
+    if (cleanState) {
+      // Abort any outstanding ams.
+      this.runGraceful(['am', '--abort'], {stdio: 'ignore'});
+      // Abort any outstanding cherry-picks.
+      this.runGraceful(['cherry-pick', '--abort'], {stdio: 'ignore'});
+      // Abort any outstanding rebases.
+      this.runGraceful(['rebase', '--abort'], {stdio: 'ignore'});
+      // Clear any changes in the current repo.
+      this.runGraceful(['reset', '--hard'], {stdio: 'ignore'});
+    }
+    return this.runGraceful(['checkout', branchOrRevision], {stdio: 'ignore'}).status === 0;
+  }
+
   /**
    * Assert the GitClient instance is using a token with permissions for the all of the
    * provided OAuth scopes.