Optionally follow symlinks to directories

[israel-lugo]

It would be nice to have the possibility of following symbolic links to (sub)directories. Currently, we don't follow them.

find_duplicates_in_dirs could have a new follow_links parameter, defaulting to False for compatibility.

[israel-lugo]

We can implement this in index_files_by_size. It can get the follow_links value from find_duplicates_in_dirs, and pass it on to os.walk as the followlinks argument.

[israel-lugo]

This needs unit testing. Test that follow_links=False doesn't follow links, and that follow_links=True does. How do we do this portably, e.g. what to do on systems that don't support symlinks?

I suppose we could detect if the system supports symlinks somehow, and change our expectations accordingly.

[israel-lugo]

Actually, make that follow_dirlinks. What we want to influence is explicitly only links to directories, not links to files. Cf man rsync for a similar example.