fix: bug retrieving diff with custom a base sha (#945)

Co-authored-by: repo-ranger[bot] <39074581+repo-ranger[bot]@users.noreply.github.com>

diff-sha.sh

@@ -223,6 +223,10 @@ else
     echo "::debug::Previous SHA: $PREVIOUS_SHA"
   else
     PREVIOUS_SHA=$INPUT_BASE_SHA && exit_status=$? || exit_status=$?
+
+    if ! git diff --name-only --ignore-submodules=all "$PREVIOUS_SHA$DIFF$CURRENT_SHA" 1>/dev/null 2>&1; then
+        DIFF=".."
+    fi
   fi
 
   echo "::debug::Target branch: $TARGET_BRANCH"