[fix #25] RAW command on empty file

When sending a RAW command to an empty file, the answer does not contain any
SyncCommand, whereas it should have contained at least one (maybe empty) Delta()
command. This is caused by an erroneus checks in the flushStack() function,
which does not output Delta() commands.

fixes #25
Fix handling of new empty file

A new file which is empty is first synchornized with a file command
without revision (because it is new) and an empty raw command.