change the default state for 'back up some subdirectories'

now, only the first-level subdirectories have magic state: they are backed
up by default. second- and subbier don't have separate state by default,
so if the user deselects foo/src and opens foo/src to look at the
subdirectories, then those aren't autoselected.

The top-level ones remain selected because backing up everything is a
plausible default. It's better for the user's mental health to deselect
the things that should not be backed up than to select all that should.