Make install script less noisy

Before this patch, the `git submodule sync` line would always print to
stdout.

tools/git-submodule/install

@@ -9,7 +9,7 @@ DOTBOT_BIN="bin/dotbot"
 BASEDIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
 
 cd "${BASEDIR}"
-git -C "${DOTBOT_DIR}" submodule sync --recursive
+git -C "${DOTBOT_DIR}" submodule sync --quiet --recursive
 git submodule update --init --recursive "${DOTBOT_DIR}"
 
 "${BASEDIR}/${DOTBOT_DIR}/${DOTBOT_BIN}" -d "${BASEDIR}" -c "${CONFIG}" "${@}"