Pure nested phony rules do not print "no work to do"

Fix & unit test for #424

Symptom : Running ninja on a project with pure phony rules returns to the command prompt with no message.
Expected : Prints "no work to do"

Cause : The check to print "no work to do" relies on the inital response of the function more_to_do. This currently returns false as it checks wanted_edges (phony + real rules) not command_edges (real rules). 

Fix : Use command_edges > 0 for more_work_to_do, and add a missing decrement for this counter.