gitlog-to-changelog coding cookie and mv -i

gitlog-to-changelog coding cookie and mv -i

* build-aux/gitlog-to-emacslog: Use ChangeLog.1, not Makefile.in,
for copyright notice prototype, so that we get a proper "coding:"
cookie.  Use 'mv -i' to avoid unconditionally overwriting an
existing ChangeLog.  Problems reported by Eli Zaretskii in:
http://lists.gnu.org/archive/html/emacs-devel/2015-04/msg00504.html