bootstrap: Ensure shallow gnulib clone works on newer git

bootstrap: Ensure shallow gnulib clone works on newer git

Update the bootstrap script to be compatible with newer versions of git
that changed the "git clone -h" output from containing:

  --depth

to:

  --[no-]depth

This bootstrap script is pulled the latest gnulib version from gnulib
git, commit 9a1a6385 (Silence 'time-stamp' warnings with bleeding-edge
Emacs.). This change avoids a full clone on gnulib, saving something
like 50 MB.

Fixes: https://savannah.gnu.org/bugs/?66357
Signed-off-by: Andrew Hamilton <adhamilt@gmail.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>