gitlog-to-changelog: Add a new --commit-timezone option. * build-aux/gitlog-to-changelog: Use the date given in the commit time zone if --commit-timezone is used. (usage): Mention the new option. * doc/gitlog-to-changelog.texi (gitlog-to-changelog): Mention the --commit-timezone and add an invocation example. Add example of date ordering that may be undesired.