ignore files in scanFile instead of scan to catch all occurences.

This tries to fix https://github.com/owncloud/mirall/issues/465
'Tries' because I cannot reproduce the issue and have to assume it is only triggered for external mount points that get updated.

Nevertheless, files should be ignored in scanFile(), not in scan(). Therefore I split isIgnoredFile into isIgnoredFile() and isIgnoredDir() and used them in scan() and scanFile() respectively.

I also added a debug log message for ignored files that might explain why a file is not scanned.

@DeepDiver1975 @dragotin @danimo @icewind1991 please test and :+1: