Optional --child snapshot is confusing

[ceremcem]

It's confusing to omit the --child parameter because it's not clear how the CHILD snapshot is created (or what is done).

Proposal

A diff tool should not attemt to auto-generate the second reference, that's a confusing behavior. This "having optional --child parameter" feature should be removed and --child parameter should be a requirement.

[ceremcem]

Looking in the source code, it's actually required:

btrfs-snapshots-diff/btrfs-snapshots-diff.py

Lines 602 to 605 in 15512d6

	else:
	printerr('Error: parent needs child!\n')
	parser.print_help()
	exit(1)

It's only a wrong claim of README.