Use GitHub's merge head for checking out PRs

.circleci/config.yml

@@ -18,10 +18,10 @@ jobs:
           name: Install GDB
       - run:
           command: ./.circleci/run.sh setup-repos
-          name: Clone DMD & DRuntime
+          name: Clone DRuntime & Phobos
       - run:
           command: ./.circleci/run.sh coverage
-          name: Run Phobos testsuite with -cov
+          name: Run testsuite with -cov
       - run:
           command: ./.circleci/run.sh check-clean-git
           name: Check for temporary files generated by the test suite

.circleci/run.sh

@@ -74,12 +74,9 @@ setup_repos() {
     # merge testee PR with base branch (master) before testing
     if [ -n "${CIRCLE_PR_NUMBER:-}" ]; then
         local head=$(git rev-parse HEAD)
-        git fetch https://github.com/dlang/$CIRCLE_PROJECT_REPONAME.git $base_branch
+        git remote add upstream "https://github.com/dlang/$CIRCLE_PROJECT_REPONAME.git"
+        git fetch -q upstream "+refs/pull/${CIRCLE_PR_NUMBER}/merge:"
         git checkout -f FETCH_HEAD
-        local base=$(git rev-parse HEAD)
-        git config user.name 'CI'
-        git config user.email '<>'
-        git merge -m "Merge $head into $base" $head
     fi
 
     for proj in druntime phobos; do