function getRowLabelForIndex(diffs, compareItemIndex, baseItemIndex) {
  return getRowLabel(getMatchingDiffsForIndex(diffs, compareItemIndex, baseItemIndex));
}
Given the array of all diffs for an audit, determine the label for a row with particular item index.

@param {Array<LHCI.AuditDiff>} diffs
@param {number|undefined} compareItemIndex
@param {number|undefined} baseItemIndex
@return {RowLabel}