I appreciate this is probably really obvious to most but it took me a little while to figure out how to do as it's not documented.
The installation instructions are spot on if you creating a new project but I'm working on an existing one and wanted to quickly create a new branch on my existing repo and give Fuel a whirl. I also wanted to be able to keep Fuel up to date via Git if necessary rather download a zip with each update.
Here goes (within you current repo)
git symbolic-ref HEAD refs/heads/fuelphp *call the branch what you like - fuelphp*
rm .git/index
git clean -fdx *this will clean up the branch completely, removing any untracked files too!*
git remote add fuelphp git://github.com/fuel/fuel.git
git pull fuelphp develop *or master, your choice*
git submodule update --init
Done
When you want to pull the latest changes - git pull fuelphp develop or master.
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When you want to pull the latest changes - git pull fuelphp develop or master. 