diff-doc.js: put back accidentally removed variable removed in TortoiseSVN revision 23508

Based on TortoiseSVN revision 24041

Signed-off-by: Sup Yut Sum <ch3cooli@gmail.com>
diff-xls.js: objExcelApp isn't even created when neither Office nor OpenOffice
are installed so check for that instead of null

Based on TortoiseSVN revision 24042

Signed-off-by: Sup Yut Sum <ch3cooli@gmail.com>