[Bug-vc-dwim] bug#56750: modernize bootstrap.conf

[Bruno Haible]

Hi,

The 'bootstrap' script from Gnulib has been reorganized to work in two phases:
  (1) Fetch auxiliary files that are not in the git checkout.
      This is the part that requires network access and that has
      supply-chain concerns.
  (2) Generate files such as configure, config.h, Makefile.in etc.
      This includes running gnulib-tool.

See <https://lists.gnu.org/archive/html/bug-gnulib/2022-07/msg00053.html> for
details.

For GNU vc-dwim some adjustment of the bootstrap.conf is needed. The 
bootstrap.conf
is meant to contain configuration settings, that is, define variables and
functions (hooks with special meanings). Instead, vc-dwim/bootstrap.conf does 
some
actions directly. This will not work satisfactorily with the new two-phase
bootstrap. What is needed, is to move these actions into hooks:
  - Actions that may require network access belong in the
    bootstrap_post_pull_hook function.
  - Actions that merely regenerate files (without network access) usually
    belong in the bootstrap_post_import_hook.

The lines

  # Automake requires that ChangeLog exist.
  touch ChangeLog

can be moved into the bootstrap_post_import_hook function.

Bruno