How do you view a different branch of your GitHub Pages?

GitHub Pages are built on top of Jekyll, which you can also install and run locally. The short version is:

1. Install Ruby
2. Install Bundler

3. Install the version of Jekyll that GitHub uses by creating a Gemfile that contains

   source 'https://rubygems.org'
   gem 'github-pages'
   

   and running bundle install

Once that is done you can run Jekyll locally in a way that matches GitHub's setup for Pages:

Running Jekyll

To run Jekyll in a way that matches the GitHub Pages build server, run Jekyll with Bundler. Use the command bundle exec jekyll serve in the root of your repository (after switching to the gh-pages branch for project repositories), and your site should be available at http://localhost:4000. For a full list of Jekyll commands, see the Jekyll documentation.

I believe you'll want to do this from your mobile branch.

One option might be to fork your primary repo to a secondary one (call it beta.yourdomain.com) and then build there and test on the beta domain, before pull-requesting the final version back into your master repo.

TL;DR - don't use a branch, use a forked repo and a subdomain to preview the build :)