merge_helpers.sh: downstream_latest_tag: Refer to HEAD instead of sc

Fix getting the best tag in detached head state, when sc is not
up-to-date.

Change-Id: I50294b9de3134815deef30a4b1c61da81721de89

merge_helpers.sh

@@ -28,7 +28,7 @@ upstream_previous_tag() {
     git describe --abbrev=0 `upstream_latest_tag`~1 --tags
 }
 downstream_latest_tag() {
-    local commit="sc"
+    local commit="HEAD"
     while true; do
         local tag=`git describe --abbrev=0 "$commit" --tags`
         if [[ "$tag" =~ "sc_" ]]; then