bootstrap: Implement phase 1 as documented in the --help output.
Reported by Simon Josefsson as bug #1 in
<https://lists.gnu.org/archive/html/bug-gnulib/2024-04/msg00152.html>.
* top/bootstrap: Correct indentation.
* top/bootstrap-funclib.sh (prepare_GNULIB_SRCDIR): Implement as
documented:
If GNULIB_SRCDIR and GNULIB_REVISION are set and there is a 'gnulib'
submodule, checkout the revision GNULIB_REVISION.
If GNULIB_SRCDIR and GNULIB_REVISION are set and --no-git is specified,
don't checkout the revision GNULIB_REVISION.