On the interface, I would suggest renaming 'exclude' to 'ignore' for 
consistency with filecmp.dircmp. Also consider detecting file separator 
in the patterns and interpreting them as an absolute (if 
pattern.startswith(pathsep)) or relative with respect to src. 

On the implementation, consider making 'exclude_files' a set for a 
faster lookup.   It should also be possible to refactor the code to 
avoid checking the type of 'exclude' on every file and every recursion.