Add ci task to fail on code-style (yapf)

ci/tasks/style.yaml

@@ -3,11 +3,13 @@ platform: linux
 inputs:
   - name: quay-pull-request
 run:
-  path: /bin/sh
+  path: /bin/bash
   args:
     - -c
     - |
       set -eux
       pip install --quiet -r quay-pull-request/requirements-dev.txt
       cd quay-pull-request
-      yapf -d -r .
+      export GIT_MERGE_BASED=`git merge-base origin/master HEAD`
+      export MODIFIED_FILES=`git diff --name-only $(GIT_MERGE_BASED) | grep -E .+\.py$ | paste -sd ' '`
+      if [ `yapf -d -p $(MODIFIED_FILES) | wc -l` -gt 0 ] ; then false ; else true ;fi