Added class EmptyDirectorySnapshot to fix issue #612.

[Ajordat]

I've added the class EmptyDirectorySnapshot.

This class is used to fake a DirectorySnapshot of an empty directory. The utility of this is to detect the files and folders of an already-with-content directory as just created.

This can be useful if the application needs to process all the files ignoring if they were created before or after the start of the application.

For example, if the folder has the following structure:

├── folder
│   └── file_in_folder.txt
├── file_a.txt
└── file_b.txt

If we run the following lines of code:

current_snapshot = DirectorySnapshot('/')
diff = DirectorySnapshotDiff(EmptyDirectorySnapshot(), current_snapshot)

The resulting diff will comply with the following asserts:

assert diff.files_created = ['/file_a.txt', '/file_b.txt', '/folder/file_in_folder.txt']
assert diff.dirs_created = ['/', '/folder']

[BoboTiG]

I like the idea, great work @Ajordat !

Could you add a line in changelog.rst and it will be perfect :)

[BoboTiG]

And let me know when the PR is good to review ;)

[Ajordat]

Done! I think I've finished, review it whenever you want :)

[BoboTiG]

Great PR, thanks a lot! 💪

[scheung38]

Thanks @BoboTiG and @Ajordat for being so responsive