set-git-source.sh incorrectly assumes `master` branch as starting point

[osowski]

The set-git-source.sh script incorrectly assumes that it will always be replacing master as the git base branch.

This is problematic if you create a new branch (newBranch1), run the script, push your changes, and deploy. Then you want to create a new environment branch based off of your current branch (newBranch2), run the script again, push your changes, and deploy. But all your files are still pointing to newBranch1 as the targetRevision in the ArgoCD Application YAMLs.

Reference: https://github.com/cloud-native-toolkit/multi-tenancy-gitops/blob/master/scripts/set-git-source.sh#L27