Avoid recloning the repo when generating docs

Author: bcylin

Makefile

@@ -24,6 +24,7 @@ carthage:
 	set -o pipefail && carthage build --no-skip-current --verbose | bundle exec xcpretty
 
 docs:
-	rm -rfv docs
-	git clone -b gh-pages --single-branch https://github.com/polydice/ICInputAccessory.git docs
+	test -d docs || git clone -b gh-pages --single-branch https://github.com/polydice/ICInputAccessory.git docs
+	cd docs && git fetch origin gh-pages && git clean -f -d
+	cd docs && git checkout gh-pages && git reset --hard origin/gh-pages
 	bundle exec jazzy --config .jazzy.yml