Fix Circle merge on master branch.

There exists a PR on a random fork, which fills in `CI_PULL_REQUEST`,
which we don't want to use as merge target when building `master`.
Instead, check `CIRCLE_PR_NUMBER`, which is set only on forked PRs.
Also, check branch name (which is current, not the _target_) to be
certain.

Author: QuLogic

.circleci/config.yml

@@ -31,10 +31,10 @@ commands:
               git remote add upstream git://github.com/matplotlib/matplotlib.git
             fi
             git fetch upstream
-            export merge=${CI_PULL_REQUEST//*pull\//}
-            if [[ "$merge" != "" ]]; then
-              echo "Merging ${merge}";
-              git pull --ff-only upstream "refs/pull/${merge}/merge";
+            if [[ "$CIRCLE_BRANCH" != "master" ]] && \
+               [[ "$CIRCLE_PR_NUMBER" != "" ]]; then
+              echo "Merging ${CIRCLE_PR_NUMBER}"
+              git pull --ff-only upstream "refs/pull/${CIRCLE_PR_NUMBER}/merge"
             fi
 
   apt-install: