bootstrap: make ensure_changelog work for update mode projects.

* gl/build-aux/bootstrap.in (func_bootstrap): Move
func_changelog_ensure invocation from here...
(func_reconfigure): ...to here. If gnulib_modules is empty, then
grep gnulib-cache.m4 for gitlog-to-changelog before invoking
func_changelog_ensure.
* bootstrap: Regenerate.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>