Create a script to rebase 5_branch and 4_branch against AS7 master

Rebasing against AS7 master needs to be done regularly, especially when you are making a change that is dependent on a new commit from upstream.

Doing the rebase is dangerous as it needs a -f. By scripting this, we can hopefully make the operation safer. This is providing the script is reviewed carefully.

We may also want the script to take a backup of 5_branch prior to the forced update, so that any error can be easily reverted.