[ci] clone dumux source branch

.gitlab-ci/default.yml

@@ -28,9 +28,9 @@ select tests:
           echo "Source branch: $DUMUX_MR_SOURCE_BRANCH"
           echo "Target branch: $DUMUX_MR_TARGET_BRANCH"
 
-          git clone -b ${DUMUX_MR_TARGET_BRANCH} --depth 1 $DUMUX_REPO
+          git clone -b ${DUMUX_MR_SOURCE_BRANCH} --depth 1 $DUMUX_REPO
           pushd dumux
-              git fetch --depth=1 origin ${DUMUX_MR_SOURCE_BRANCH}:${DUMUX_MR_SOURCE_BRANCH}
+              git fetch --depth=1 origin ${DUMUX_MR_TARGET_BRANCH}:${DUMUX_MR_TARGET_BRANCH}
           popd
 
       else