status: lose dependence on StringIO

buflist was being used, which isn't available in Python 3.

`Execute` was using StringIO to capture the output of `PrintWorkTreeStatus`,
only to redirect it straight to stdout.
Instead, just let `PrintWorkTreeStatus` do it's own thing directly to stdout.

for handling `_FindOrphans`, we swap StringIO for a list. Nothing was done
that needed a a file like object.

Change-Id: Ibdaae137904de66a5ffb590d84203ef0fe782d8b