If an important file is changed, we regenerate version.ent and so bump
the revision. Otherwise, version.ent may be kept. Hack git-version.sh
so we can ensure version.ent is really not changed.
Add Makefile itself and git-version.sh into the important file list as
they can definitely affect book rendering.
There was some consistency issues in URLs to LFS. For example, in
systemd revision the URL to LFS "General Network Configuration" points
to the sysv book, this can be really troubling to new readers.
Instead of fixing them one by one, merge conditional XML developed by
Pierre from LFS.
