Change diffing logic for the new release process.

[lina128]

To prepare for the new release process. Change the diffing logic so that if PR is sent to a release branch, diff against that branch, not master.

---

This change is 

[dsmilkov]

Reviewed 2 of 2 files at r1.
Reviewable status: complete! 1 of 1 approvals obtained (waiting on @dsmilkov, @lina128, @nsthorat, and @pyu10055)

---

cloudbuild.yml, line 17 at r1 (raw file):

  - 'COMMIT_SHA=$COMMIT_SHA'
  - 'BRANCH_NAME=$BRANCH_NAME'
  - 'BASE_BRANCH=$_BASE_BRANCH'

just confirming that _BASE_BRANCH is pre-populated by cloudbuid, is that right?

[lina128]

Reviewed 2 of 2 files at r1.
Reviewable status: complete! 1 of 1 approvals obtained (waiting on @dsmilkov, @lina128, @nsthorat, and @pyu10055)

cloudbuild.yml, line 17 at r1 (raw file):

  - 'COMMIT_SHA=$COMMIT_SHA'
  - 'BRANCH_NAME=$BRANCH_NAME'
  - 'BASE_BRANCH=$_BASE_BRANCH'

just confirming that _BASE_BRANCH is pre-populated by cloudbuid, is that right?

Hi Daniel, that is right, it is documented here: https://cloud.google.com/cloud-build/docs/configuring-builds/substitute-variable-values
And I tested here, see CI log: https://pantheon.corp.google.com/cloud-build/builds/96e2d6be-b872-4ea8-8872-c1f211157a81?project=learnjs-174218