fix(deploy): clean up gh-pages obsolete files (angular#3081)

Prevents the gh-pages branch from growing indefinitely by cleaning up before copying new files.
Fixes angular#3081

Author: rolyatsats

packages/angular-cli/commands/github-pages-deploy.ts

@@ -147,6 +147,7 @@ const githubPagesDeployCommand = Command.extend({
       .then(saveStartingBranchName)
       .then(createGitHubRepoIfNeeded)
       .then(checkoutGhPages)
+      .then(cleanGhPagesBranch)
       .then(copyFiles)
       .then(createNotFoundPage)
       .then(addAndCommit)
@@ -205,6 +206,20 @@ const githubPagesDeployCommand = Command.extend({
         .then(() => execPromise(`git commit -m \"initial ${ghPagesBranch} commit\"`));
     }
 
+    function cleanGhPagesBranch() {
+      return execPromise('git ls-files')
+        .then(function(stdout) {
+          var files = '';
+          stdout.split("\n").forEach(function(f){
+            // skip .gitignore & 404.html
+            if(( f != '') && (f != '.gitignore') && (f != '404.html')){
+              files = files.concat(`"${f}" `);
+            }
+          });
+          return execPromise(`git rm -r ${files}`);
+        });
+    }
+
     function copyFiles() {
       return fsReadDir(outDir)
         .then((files: string[]) => Promise.all(files.map((file) => {