Different triggers for PR merged into different branches

[nickslavsky]

Our workflow process involves first merging changes into the staging environment and only after that - to prod. We'd like to automate the respective ticket transitions between the states. For example: 

• code review -> available on staging - when the code is merged into staging
• available on staging -> available on prod - when the code is merged into master

However, we found that the bitbucket triggers lack granularity: when the same code is merged into the master branch, the same trigger fires again causing the same transition.

What's the best way to achieve our desired behavior?