Step 1: Follow this tutorial
Step 2: Set up your git pre-commit hook by following this tutorial
Step 3: With this set up, you would add a new post simply by saving your post in your content folder, committing from your source directory (wherever your pelicanconf.py file is located), and then just pushing to origin.
Woohoo!
Basically you have two git repositories: one for your blog-source (the main Pelican directory where your publishconf.py file lives), and one for your output folder, which is rendered, updated and pushed to your GitHub output folder every time you push your commits from blog-source to your GitHub blog-source folder.