fix check if file has changed when md5 is used
  * only individual snapshots where compared
  * if a file was restored, 'FilterWhereFileWasModified' was incorrect
    - it has versions listed with the same content as the actual file
  * now the actual file md5 a compared too
new compare-file-method: auto
  * use md5 on text files
  * size+modTime on others
  * is the new default
update README
show current snapshot name in file-diff view
return empty Diff if no differences found
  * go-diff returns a diff with one equal element when the files a equal