AtlasBuildScripts: add atlas_git_merge script

This is a first version of a script to be used by the RC to merge changes from the production branch into main. It would replace the use of the merge driver in our current instructions. The merge driver is not really suitable for our use-case at it only rejects changes to e.g. Projects/ in case there is a merge conflict. This has caused problems and confusion a few times in the past.

Usage:

atlas_git_merge.sh 24.0

@atlas-release-coordinators @jcatmore @nstyles @jchapman

I used it this week and I invite @tadej to try it next week.