enhance logging for depend tracking mode

Add some extra log, to make the console output more understandable

Change-Id: If80cb9dba1031ad268bf9b6266ca83f93071a2ad
diff --git a/stack.sh b/stack.sh
index 36312ea..f6c4824 100755
--- a/stack.sh
+++ b/stack.sh
@@ -757,6 +757,7 @@
 if [[ $TRACK_DEPENDS = True ]]; then
     $DEST/.venv/bin/pip freeze > $DEST/requires-post-pip
     if ! diff -Nru $DEST/requires-pre-pip $DEST/requires-post-pip > $DEST/requires.diff; then
+        echo "Detect some changes for installed packages of pip, in depend tracking mode"
         cat $DEST/requires.diff
     fi
     echo "Ran stack.sh in depend tracking mode, bailing out now"