changed: do not always start update_data branch from origin/master

this allows for update_data from opm-tests. the generated
PR will include the contents of the PR it was generated for

tests/update_reference_data.sh

@@ -246,7 +246,7 @@ done
 cd $OPM_TESTS_ROOT
 if [ -n "$BRANCH_NAME" ]
 then
-  git checkout -b $BRANCH_NAME origin/master
+  git checkout -b $BRANCH_NAME $BRANCH_BASE
 fi
 
 # Add potential new files