Create the ChangeLog from the git commit log.
* autogen.sh: Install git commit log checker. * build-aux/git-hooks/commit-msg: New. Repo only. * build-aux/git-log-fix: New. Repo only. * build-aux/git-log-footer: New. Repo only. * build-aux/gitlog-to-changelog: New. Repo only. * HACKING: New. * Makefile.am (gen-Changelog): New rule. (dist-hook): Depend on gen-Changelog. (EXTRA_DIST): Add HACKING. * ChangeLog: New stub file.