fix: fail when the merge base is not found (#879)

diff-sha.sh

@@ -270,12 +270,12 @@ else
 
         # shellcheck disable=SC2086
         git fetch $EXTRA_ARGS -u --progress --deepen="$i" origin $TARGET_BRANCH $CURRENT_SHA 1>/dev/null 2>&1
+      done
 
       if ((i > max_depth)); then
         echo "::error::Unable to locate a common ancestor between $TARGET_BRANCH and $CURRENT_BRANCH with: $PREVIOUS_SHA$DIFF$CURRENT_SHA"
         exit 1
       fi
-      done
     else
       echo "::debug::Not a shallow clone, skipping merge-base check."
     fi