new bootstrap script

[Jim Meyering]

Hi again,

I've just updated the bootstrap script by merging in
several months worth of changes that have been tested
in coreutils and a few other gnulib-based projects.

Among the new features:

  - pulls gnulib updates using git rather than CVS if you don't
      specify --gnulib-srcdir=YOUR_GNULIB_DIR

  - gets .po file updates via rsync at the expense of saving
      local copies in po/.reference/;  this means that there's
      no longer a reason to run bootstrap with its --skip-po
      option, because updating po files is so fast now.

  - slightly better .gitignore support

and probably a few more i don't remember off hand