Provide a hook to fix the changelog

The Gitlab changelog feature requires a linear history from one tagged
release to the next - which is not always the case. Provide a script
to allow repositories to adjust the changelog depending on their branch
strategy (the default implementation does nothing).

After the script is run, the last commit is amended if the changelog has
been modified.

Changelog: other
Signed-off-by: Adam Johnston <adam.johnston@linaro.org>