Remove unused imports.
Invoke static method using class from static method.
Fix reference to member variable "goal".
Remove unused local variable.
Merge branch 'pyflakes'

Fix up several coding errors discovered by pyflakes.
Extract a method Block.fill_diagram().
Extract a method MergeFrontier.fill_diagram().
Merge branch 'fill-diagram'

Generate diagram output in two stages:

* Write one string for each commit to a 2D array
* Write the array to output

Then give access to the former functionality via new fill_diagram()
methods.
Make Block.set_value() private and remove Block.__setitem__().

Callers shouldn't be setting records like this.  They should get the
MergeRecord using __getitem__() and then modify the MergeRecord in
place.