all commands now treat a nil selection as their preferred default

Some commands operate on the whole repository, others bail with an error message, etc.

We avoid setting rs.selection to any particular value, preferring to interpret the nil with some meaning, but leave the nil in place. The primary benefit at the moment is that we never set it to repo.all() and then turn around and compare it with the result of calling repo.all() again. That comparison is particularly slow, and DoWrite in particular was suffering as a result.

fixes #162 (closed)