Print test diffs into GitHub summary

So that we can also observe them for try builds, before merging a PR.

.github/workflows/ci.yml

@@ -252,7 +252,12 @@ jobs:
             exit 0
           fi
 
+          # Get closest bors merge commit
+          PARENT_COMMIT=`git rev-list --author='bors <bors@rust-lang.org>' -n1 --first-parent HEAD^1`
+
           ./build/citool/debug/citool postprocess-metrics \
+              --job-name ${CI_JOB_NAME} \
+              --parent ${PARENT_COMMIT} \
               ${METRICS} >> ${GITHUB_STEP_SUMMARY}
 
       - name: upload job metrics to DataDog