improve user interface for shallow working copies

Sparse working copies can easily cause user confusion. E.g. they can cause
skipped subtrees during merges and it might not be immediately apparent to users
why the subtree is skipped (see issue #3618).
Users can easily forget about shallow subtrees because, currently, the only way
to know if a working copy is shallow is parsing the output of svn info -R.
If svn status clearly marked directories that are roots of shallow subtrees as
such this problem would be mitigated.

For example, we could use a notation similar to how tree conflicts are presented
to show ambient depths smaller than 'infinity' in the status output:

$ svn status
D       gamma/delta
        epsilon
        > depth 'empty'
M       alpha
$

In the above example, epsilon has depth 'empty'.