bootstrap: Fix recognition of --depth option with recent git releases. Reported by Benno Schulenberg <bensberg@telfort.nl> in <https://lists.gnu.org/archive/html/bug-gnulib/2025-02/msg00106.html>. * top/bootstrap-funclib.sh (prepare_GNULIB_SRCDIR): Assume that 'git clone' and 'git fetch' support the --depth option. * build-aux/bootstrap: Regenerated.