add a pre-commit hook that check that files are properly gofumpt-ed

.githooks/pre-commit

@@ -13,6 +13,14 @@ for f in $(git diff --cached --name-only); do
 	fi
 done
 
+# Check that all files are properly gofumpt-ed.
+output=$(gofumpt -d $(git diff --cached --name-only))
+if [ -n "$output" ]; then
+	echo "Found files that are not properly gofumpt-ed."
+	echo "$output"
+	errored=true
+fi
+
 if [ "$errored" = true ]; then
 	exit 1
 fi