maint: make ChangeLog generation more robust

* Makefile.am (gen-ChangeLog): Check the git-log-fix file is present
before trying to use it.  This change was made to coreutils by Pádraig
Brady and copied to GNU Hello as-is.
* build-aux/git-log-fix: Add example how to use git-log-fix file.
update gnulib

* gnulib: jump 56 commits a head