clean up buildkite output

.buildkite/merge_base_branch.sh

 #!/usr/bin/env bash
 
-set -ex
+set -e
 
 if [[ "$BUILDKITE_BRANCH" =~ ^(develop|master|dinsic|shhs|release-.*)$ ]]; then
     echo "Not merging forward, as this is a release branch"
@@ -18,6 +18,8 @@ else
     GITBASE=$BUILDKITE_PULL_REQUEST_BASE_BRANCH
 fi
 
+echo "--- merge_base_branch $GITBASE"
+
 # Show what we are before
 git --no-pager show -s