Prevent retesting the same commit (based on hash)

[Aaron Hong]

I have tests running on the `default` pipeline, so it runs both on pull request branches and our `master` branch. It's fine most of the time. However, when I fast-forward merge a pull request, the exact same code gets tested twice, once from the pull request and again on `master`. Same thing happens when I create a branch from `master`. I don't want to waste my pipeline minutes. Is there a way to prevent that?

Fast-forward merge scenario

• #91 My commit message (h4sh) <-- master branch
• #90 My commit message (h4sh) <-- feature branch

New feature branch scenario

• #98 My commit message (h4sh) <-- new feature branch
• #97 My commit message (h4sh) <-- master branch