Once you clone it, you can keep it updated with the following commands (these assume that darktable sources are in ~/darktable):
cd ~/darktable
git clean -d -f -x
git pull --rebase
git submodule update
./build.sh <whatever params you want>
rm -rf ~/.cache/darktable/cached_kernels*
git clean -d -f -x makes sure you start with a new build environment (no left-over files from the previous build).
git pull --rebase updates the code; git submodule update updates the submodules.
rm -rf ~/.cache/darktable/cached_kernels* removes the compiled versions of OpenCL kernels, to make sure you start with a clean slate there as well.