bootstrap: Support checking out a recent GNULIB_REVISION, part 2. Reported by Markus Mützel <markus.muetzel@gmx.de> in <https://lists.gnu.org/archive/html/bug-gnulib/2024-04/msg00507.html>. * top/bootstrap-funclib.sh (prepare_GNULIB_SRCDIR): If using a submodule and the 'git checkout' command fails, fetch the newer commits and then retry it. * build-aux/bootstrap: Regenerated.