[ci]: Support code diff coverage (#116)

Support code diff coverage, and set threshold to 50%

azure-pipelines.yml

@@ -19,6 +19,9 @@ stages:
  - job:
  displayName: "build"
  timeoutInMinutes: 60
+ variables:
+ DIFF_COVER_CHECK_THRESHOLD: 50
+ DIFF_COVER_ENABLE: 'true'
  pool:
  vmImage: ubuntu-20.04
 
@@ -52,13 +55,6 @@ stages:
  sudo python3 -m pip install dist/swsssdk-2.0.1-py3-none-any.whl
  python3 -m pytest --doctest-modules --junitxml=junit/test-results.xml --cov=. --cov-report=xml
  displayName: "Unit tests"
- - script: |
- sudo pip3 install diff-cover
- target_branch=$(System.PullRequest.TargetBranch)
- compare_branch=origin/${target_branch#refs/heads/}
- diff-cover coverage.xml --compare-branch=$compare_branch
- condition: eq(variables['Build.Reason'], 'PullRequest')
- displayName: "Diff coverage"
  - publish: $(Build.ArtifactStagingDirectory)
  artifact: sonic-py-swsssdk
  displayName: "Archive artifacts"