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
Revert "maint: make ChangeLog generation more robust"

I should have used patch sent by Pádraig Brady to GNU Hello maillist.

This reverts commit fb1c5fbd5eee3ab6e3270b6aaf4037bca8ab20bd.
build: make git-log-fix optional in make dist

* Makefile.am (gen_changelog): Make git-log-fix optional.
Also only remove existing Changelog on successful completion
of gitlog-to-changelog and propogate exit status appropriately.
maint: add an example how to use git-log-fix file

* build-aux/git-log-fix: ditto