The changelog will now be automatically updated when a release branch
is created. A new release header along with changelog entries for any
new commits will be added.

Note that this changelog will still need to be manually cleaned up, but
it's one less manual step at least.

The old Bash script for adding a new release header to the changelog
has been removed, as that functionality is now built into the changelog
update script.

A new script has been added to commit any changes made to the manifest
and changelog. This step used to happen at the end of the bump manifest
version script, but now the changelog update relies upon the manifest
version bump happening first, so it needed to be re-ordered. The
changes should only be committed on the first run of the branch, as
it's contingent upon the manifest changing (due to the version bump).
Further changelog updates won't trigger new automatic commits.

The check for manifest changes was wrong on two counts. Firstly it was
supposed to check that there *are* changes, but instead it was bailing
if there were *no* changes (the opposite of what was intended). Second,
it was including changelog changes as well, when it should have only
considered manifest changes.