update_copyright_year.sh can be smarter to exclude changesets only to previous copyright updates.

It would be helpful to be able to run copyright update checks for previous years. It would therefor be required that update_copyright_year.sh not consider changesets which contain only copyright year updates as requiring another year update.

For example, if I run 'update_copyright_year.sh 2013' but commit during 2014, a changeset with those edits to the copyright date (and only copyright updates) would appear to a subsequent run of 'update_copyright_year.sh 2014' as requiring the year updated to 2014. Such changesets (only containing year updates) should be excluded by the script.