tools/format-cpp.sh: ignore `.git` directory

tools/format-cpp.sh: ignore `.git` directory

A `.hpp` or `.cpp` file could exist in `.git`, for example in
`.git/logs/refs/heads` if the branch name ends with `.hpp` or `.cpp`.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I328486a81148fe46221bac8979ab6127ed4e3903
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11707
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>