Update diff-sha.sh

diff-sha.sh

@@ -221,7 +221,7 @@ else
         echo "Fetching $depth commits..."
 
         # shellcheck disable=SC2086
-        git fetch $EXTRA_ARGS -u --progress --deepen="$depth"
+        git fetch $EXTRA_ARGS -u --progress --deepen="$depth" origin +"$GITHUB_REF":refs/remotes/origin/"$CURRENT_BRANCH"
 
         if [[ $depth -gt $max_depth ]]; then
           echo "::error::Unable to locate a common ancestor between $TARGET_BRANCH and $CURRENT_SHA"