Use base revision provided by CircleCI (#2309)

instead of GitHub API

Co-authored-by: Kevin Barabash <kevinb@khanacademy.org>

.circleci/config.yml

@@ -19,21 +19,6 @@ commands:
           command: |
               git submodule sync
               git submodule update --init --recursive
-      - run:
-          name: Get changed files
-          command: |
-              if [[ $CIRCLE_PULL_REQUEST ]]; then
-                BASE_COMMIT=$(curl -s -H "Authorization: token a61ecb2fc5b72da54431""1b3db3875c96854958a8" \
-                  https://api.github.com/repos/$CIRCLE_PROJECT_USERNAME/$CIRCLE_PROJECT_REPONAME/pulls/$CIRCLE_PR_NUMBER \
-                  | jq -r ".base.sha | select(. != null)")...
-              else
-                BASE_COMMIT=HEAD^
-              fi
-              if [[ $BASE_COMMIT ]]; then
-                CHANGED=$(git diff --name-only $BASE_COMMIT)
-                echo "$CHANGED"
-                echo "export CHANGED=\"$CHANGED\"" >> $BASH_ENV
-              fi
 
   skip_if_only_changed:
     parameters:
@@ -42,7 +27,7 @@ commands:
     steps:
       - run:
           name: Skip tests if only "<< parameters.filter >>" are changed
-          command: echo "$CHANGED" | grep -qvE '<< parameters.filter >>' || circleci step halt
+          command: git diff --name-only << pipeline.git.base_revision >>... | grep -qvE '<< parameters.filter >>' || circleci step halt
 
   install_dependencies:
     steps: