Report changed inputs to tasks

[jasonwhite]

When the build system executes tasks, it should give a list of modified inputs to the task.

This is useful for tasks that are recomputing which dependencies were modified. The prime example of this is with recursive builds. When run recursively, Button will compute its modified dependencies even though the parent build system already did that work.

This information is not currently available because it is encapsulated in the graph traversal algorithm. A way to extract this from the graph traversal will be needed. Currently, only the number of edges that caused a vertex traversal is given in the traversal callback.