dgit: Reorganise cleaning in build_prep

1. Replace the open-coded $includedirty with the equivalent
building_source_in_playtree(), because the latter is what we actually
care about here.

2. Reformat into if blocks.

3. Introuce and call clean_tree_check (currently a no-op).  Now,
writing clean_tree_check will be part of fixing #910705.

No functional change.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>