Test-infra: Prow runs postsubmit jobs on branch that contain name of the target branch

[atakanyenel]

What happened:
we have a postsubmit job defined to release branch. To create new release, I created a branch called release-0.1, made a commit and created a PR to release branch. Prow took my commit to release-0.1 branch as post submit and ran the job defined for the release branch.

What you expected to happen: I expected prow to run post-submit job only with exactly matching branches.

How to reproduce it (as minimally and precisely as possible): Have a post-submit job defined for master. Create a branch called master-test and push it. Prow will run the post submit job defined for master for master-test branch.

Anything else we need to know?:
We believe it's because this line does checks for contains instead of exact matching.

I believe this is a security issue as our release branch is protected by Github but now this protection is bypassed.

[fejta]

^release$ will only match the release branch. release is equivalent to ^.*release.*$