There's a script to update copyright years for all files inside changesets in a given branch:
bin/update_copyright_year.sh

Unfortunately, this is not always ideal for development branches: development branches may contain changes to files that are reverted to the original state later in that branch. As commits inside development branches are squashed at the end, these files are not updated in the final commits, and the copyright headers should not be updated.

My proposal here is to augment bin/update_copyright_year.sh with an ability to update the years for a given list of files. It could then be used like e.g.:

$ git diff upstream/master...<branch-name> | lsdiff | cut -d '/' -f 2- | bash bin/update_copyright_year.sh -m