diff: do not show 'No changes' panel when 'Can not show diff' should be shown

* show 'No Changes' as label on the top instead of replacing documents by empty panel
* remove CR/LF information from 'Can not show diff' panel
(cherry picked from commit 88ed475)
diff: do not show 'contents identical' if we can't calculate diff

ex: modified image file
(cherry picked from commit 188e321)