Since we moved to Git from Subversion we've abandoned the ChangeLog
file in favour of a more detailed commit log.

In order to maintain a ChangeLog for users of the tarballs, we need
to generate a ChangeLog file out of the commit log when distchecking
a Clutter release.

For this reason, we use a simple Perl script that is invoked by the
dist-hook and generates a ChangeLog file starting from the previous
stable release.